https://wiki.haskell.org/api.php?action=feedcontributions&user=MikeLedger&feedformat=atomHaskellWiki - User contributions [en]2024-03-29T09:26:10ZUser contributionsMediaWiki 1.35.5https://wiki.haskell.org/index.php?title=Applications_and_libraries/Games&diff=45791Applications and libraries/Games2012-05-26T23:07:43Z<p>MikeLedger: /* Games */ added Level 0</p>
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<div>{{LibrariesPage}}<br />
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See also: [[Game Development]]<br />
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== Games ==<br />
<br />
See also the [http://hackage.haskell.org/packages/archive/pkg-list.html#cat:game Game] category on Hackage.<br />
<br />
; [http://folk.uio.no/carljsv/gorillabas/GorillaBAS-0.1.tar.gz GorillaBAS]<br />
: A concrete game from an attempt on defining computer games.<br />
<br />
;[http://hackage.haskell.org/package/babylon babylon]<br />
: An implementation of a simple 2-player board game. Uses wxHaskell.<br />
<br />
;[http://hackage.haskell.org/package/boomslang boomslang]<br />
: A clone of the popular Flash game Boomshine.<br />
<br />
;[https://github.com/yairchu/defend/tree Defend The King from Forces of Different]: A simple multiplayer real time strategy game.<br />
<br />
; [http://www.increpare.com/2008/11/endless-cavern/ Endless Cavern]: A 2D procedurally-generated cave exploration game.<br />
<br />
;[http://sourceforge.net/projects/fooengine/?abmode=1 Foo]<br />
:Foo (abbreviation from football) is a playing machine of [http://en.wikipedia.org/wiki/Paper_Soccer Paper Soccer], a pencil and paper game for two players. It contains a simple interface using HOpenGL library and provides many playing algorithms.<br />
<br />
;[[Frag]]<br />
:Frag is a 3D first person shooting game written in Haskell, by Mun Hon Cheong. It uses Yampa, Quake 3 BSP level format and OpenGL. It is licensed under the GPL.<br />
<br />
;[[GeBoP]]<br />
:The General Boardgames Player, offers a set of board games: Ataxx, Bamp, Halma, Hex, Kram, Nim, Reversi, TicTacToe, and Zenix. It uses wxHaskell.<br />
<br />
;[http://www.informatik.uni-bremen.de/~cxl/lehre/pi3.ws01/asteroids/ Haskell in Space]<br />
:An asteroid like game<br />
<br />
;[http://www.hedgewars.org/ Hedgewars]<br />
:A turn-based artillery game. The game server is written in Haskell.<br />
<br />
;[http://www.cs.ox.ac.uk/people/ian.lynagh/Hetris/ Hetris]<br />
:ASCII tetris in Haskell<br />
<br />
;[http://hackage.haskell.org/package/hfiar hfiar]<br />
:Four in a Row in Haskell. Uses wxHaskell.<br />
<br />
;[http://hackage.haskell.org/package/hinvaders hinvaders]<br />
:A simple ANSI-graphics space invaders written entirely in Haskell 98.<br />
<br />
;[http://mu.org/~mux/LambdaChess/ LambdaChess]<br />
:GTK chess client<br />
<br />
;[http://www.ncc.up.pt/~pbv/stuff/lostcities/ Lost Cities]<br />
:A two-player card game where each player tries to mount profitable expeditions. It uses wxHaskell.<br />
<br />
;[http://hackage.haskell.org/package/mage Mage]<br />
:Nethack clone written in Haskell (The web site have [http://www.scannedinavian.com/~shae/mage-1.0pre35.tar.gz this mage-1.0.pre35.tar.gz file] containing an older version that was using Data.FiniteMap.) There seems to be a problem with newer curses library even with the more recent 1.1.0 version.<br />
<br />
;[http://hackage.haskell.org/package/MazesOfMonad MazesOfMonad]<br />
:Role-Playing Game (influenced by Nethack), complete and fully playable. Console mode only.<br />
<br />
;[http://www.geocities.jp/takascience/haskell/monadius_en.html Monadius]<br />
:Monadius is a shoot 'em up with the selection bar power-up system for Windows, written in Haskell (now on [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Monadius-0.9.20071203 Hackage])<br />
<br />
;[http://mokehehe.blogspot.com/2009/04/super-nario-move-to-github.html Monao]<br />
:A Super Mario clone, using an SDL binding different from the one in Hackage: [https://github.com/mokehehe/monao Monao on github]<br />
<br />
;[http://berlinbrowndev.blogspot.com/2007/09/octane-mech-opengl-haskell-based-mech.html Octane Mech]<br />
:Octane Mech, OpenGL Haskell based mech game<br />
<br />
;[http://haskell-tetris.pbworks.com/w/page/16967421/Main OpenGL Tetris]<br />
:Tetris in Haskell with OpenGL<br />
<br />
;[http://www24.brinkster.com/srineet/para/para.html Paratrooper]<br />
:Paratrooper is a simple action game that runs on Windows and is written in literate Haskell.<br />
<br />
;[http://raincat.bysusanlin.com/ Raincat]<br />
:2D puzzle game featuring a fuzzy little cat (uses GLUT)<br />
<br />
;[http://roguestar.downstairspeople.org Roguestar]<br />
:Roguestar is a science fiction adventure role playing game using Haskell and OpenGL.<br />
<br />
;[http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Shu-thing Shu-thing]<br />
:A 2-D vector graphics upwards-scrolling keyboard-controlled shooter. You shoot the enemies while dodging their bullets until you reach and defeat the enemy.<br />
<br />
;[http://hackage.haskell.org/package/SpaceInvaders Space Invaders]<br />
:A video game, based on [[Yampa]]<br />
<br />
;[http://hackage.haskell.org/package/stunts stunts]<br />
:A revival of the classic racing game Stunts to serve as a non-toy-sized example for LambdaCube.<br />
<br />
;[https://github.com/nbartlomiej/tfoo Tfoo]<br />
:A simple Five in a Row game. Online, with server-sent events, deployed to [http://tfoo.herokuapp.com/ Heroku], open source.<br />
<br />
;[http://web.jfet.org/~kwantam/TriHs.tar.gz TriHs] (tar.gz)<br />
:A 1- or 2-player Tetris game using Gtk2Hs and Cairo.<br />
<br />
;[[wxAsteroids]]<br />
:Your space ship enters an asteroid belt, try to avoid collisions! wxAsteroids is based on wxHaskell.<br />
<br />
;[http://xiangqiboard.blogspot.com/2007/12/gnuxiangqi-angekndigt.html Xiangqiboard]<br />
:An implementation of xiangqi for Unix, using gtk2hs + cairo<br />
<br />
;[http://quasimal.com/projects/level_0.html Level 0]<br />
:A fun and featureful Snake II clone using SDL.<br />
<br />
=== Unfinished/in-progress games ===<br />
<br />
;[https://github.com/Mikolaj/Allure Allure of the Stars]<br />
:A near-future Sci-Fi roguelike and tactical squad game. Long-term goals are high replayability and auto-balancing through procedural content generation and persistent content modification based on player behaviour. The game is written using the [http://hackage.haskell.org/package/LambdaHack LambdaHack] roguelike game engine.<br />
<br />
;[http://ipwnstudios.com/node/4 Bloodknight]<br />
:An action RPG for mobile devices<br />
<br />
; [https://github.com/ghulette/haskell-game-of-life haskell-game-of-life]<br />
: Conway's Game of Life<br />
<br />
;[http://dotat.at/prog/life/hslife.hs HsLife]<br />
:A Haskell implementation of hashlife. It uses GLUT.<br />
<br />
;[http://joyridelabs.de/game/ Nikki and the Robots]<br />
:A puzzle, platformer game<br />
<br />
== Game Engines and Libraries ==<br />
<br />
;[http://hackage.haskell.org/package/FunGEn FunGEn]<br />
:FunGEn (Functional Game Engine) is a platform-independent BSD-licensed 2D game engine based on OpenGL and GLUT. Its light dependencies make it easy to install, however GLUT is reputed to be unsuitable for simultaneous keypresses. As of 2011 it's the only general-purpose game engine, and the quickest way to throw together [http://darcsden.com/sordina/fungen/browse/examples/helloworld.hs simple] [http://darcsden.com/sordina/fungen/browse/examples/pong/pong.hs 2D] [http://darcsden.com/sordina/fungen/browse/examples/worms/worms.hs games], in Haskell. Forks and patches welcome!<br />
<br />
;[http://hackage.haskell.org/package/GLFW-b GLFW-b]<br />
:Bindings to GLFW, a free, open source, multi-platform library for creating OpenGL contexts and managing input, including keyboard, mouse, joystick and time.<br />
<br />
;[http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Hipmunk Hipmunk]<br />
:Hipmunk: A Haskell binding for [http://chipmunk-physics.net/ Chipmunk]. Chipmunk is a fast, simple, portable, 2D physics engine. It is completely self-contained. See also [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HipmunkPlayground HipmunkPlayground]: a simple OpenGL program that allows you to see some of Hipmunk's functions in action.<br />
<br />
;[[Hpysics]]<br />
:Hpysics is a physics engine written using Data Parallel Haskell during Google Summer of Code 2008.<br />
<br />
;[http://hackage.haskell.org/package/bullet Bullet]<br />
:A wrapper for the Bullet physics engine.<br />
<br />
;[http://projects.haskell.org/game-tree/ game-tree]<br />
:game-tree is a purely functional library for searching game trees - useful for zero-sum two player games.<br />
<br />
;[http://hackage.haskell.org/package/hogre hogre]<br />
:Haskell bindings to the excellent OGRE 3D rendering engine. Ogre has been used in commercial games such as Torchlight and several books exist documenting the Ogre API. Ogre uses an MIT license making it compatible with many Haskell libraries.<br />
<br />
;[http://hackage.haskell.org/package/IrrHaskell IrrHaskell]<br />
:Haskell binding to the [http://irrlicht.sourceforge.net/ Irrlicht game engine]. The Irrlicht Engine is an open source high performance realtime 3D engine<br />
<br />
;[http://hackage.haskell.org/package/lambdacube-engine LambdaCube]<br />
:LambdaCube is a 3D rendering engine entirely written in Haskell. The main goal of this project is to provide a modern and feature rich graphical backend for various Haskell projects, and in the long run it is intended to be a practical solution even for serious purposes.<br />
<br />
<br />
=== Unfinished/in-progress game engines/libraries ===<br />
<br />
;[[HaskGame]]<br />
:An incomplete graphics system abstraction layer.<br />
<br />
; [https://github.com/shicks/hsgame hsgame]<br />
:A framework for network games<br />
<br />
;[https://github.com/adorablepuppy/CurryDog CurryDog]<br />
:Aims to be a 2d and 3d modular game engine.<br />
<br />
;[https://github.com/kosmikus/LambdaHack LambdaHack]<br />
:A game engine library for roguelike games of arbitrary theme, size and complexity, packaged together with a small example dungeon crawler. When completed, it will let you specify content to be procedurally generated, define the AI behaviour on top of the generic content-independent rules and compile a ready-to-play game binary, using either the supplied or a custom-made main loop. Several frontends are available (GTK is the default) and many other generic engine components are easily overridden, but the fundamental source of flexibility lies in the strict and type-safe separation of code and content.<br />
<br />
<br />
<br />
[[Category:Games|*]]<br />
[[Category:Applications]]<br />
[[Category:Libraries]]</div>MikeLedgerhttps://wiki.haskell.org/index.php?title=Template_Haskell&diff=45790Template Haskell2012-05-26T22:59:56Z<p>MikeLedger: /* Other useful resources */ add a new tutorial</p>
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<div>'''[http://hackage.haskell.org/package/template-haskell Template Haskell]''' is a [[GHC]] extension to Haskell that adds compile-time metaprogramming facilities. The original design can be found here: http://research.microsoft.com/en-us/um/people/simonpj/papers/meta-haskell/ . It is [http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell.html included] in GHC since version 6. <br />
<br />
This page hopes to be a more central and organized repository of TH related things.<br />
<br />
<br />
=What is Template Haskell?=<br />
<br />
Template Haskell is an extension to Haskell 98 that allows you to do type-safe compile-time meta-programming, with Haskell both as the manipulating language and the language being manipulated. <br />
<br />
Intuitively Template Haskell provides new language features that allow us to convert back and forth between concrete syntax, i.e. what you would type when you write normal Haskell code, and abstract syntax trees. These abstract syntax trees are represented using Haskell datatypes and, at compile time, they can be manipulated by Haskell code. This allows you to reify (convert from concrete syntax to an abstract syntax tree) some code, transform it and splice it back in (convert back again), or even to produce completely new code and splice that in, while the compiler is compiling your module. <br />
<br />
For email about Template Haskell, use the [http://www.haskell.org/mailman/listinfo/glasgow-haskell-users GHC users mailing list]. It's worth joining if you start to use TH.<br />
<br />
<br />
= Template Haskell specification =<br />
<br />
Template Haskell is only documented rather informally at the moment. Here are the main resources:<br />
<br />
* [http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell.html The user manual section on Template Haskell]<br />
* [http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell.html#th-quasiquotation The user manual section on quasi-quotation], which is closely related to Template Haskell.<br />
* [http://research.microsoft.com/~simonpj/papers/meta-haskell/ The original Template Haskell paper]<br />
* [http://research.microsoft.com/en-us/um/people/simonpj/tmp/notes2.ps Notes on Template Haskell version 2], which describes changes since the original paper. Section 8 describes the difficulty with pattern splices, which are therefore not implemented.<br />
* [http://www.haskell.org/ghc/docs/7.0-latest/html/libraries/template-haskell-2.5.0.0/Language-Haskell-TH.html The Template Haskell API]<br />
<br />
= Template Haskell tutorials and papers =<br />
<br />
* Bulat's tutorials:<br />
** [http://docs.google.com/uc?id=0B4BgTwf_ng_TM2MxZjJjZjctMTQ0OS00YzcwLWE5N2QtMDI0YzE4NGUwZDM3 [formerly /bz/thdoc.htm]]<br />
** [http://docs.google.com/uc?id=0B4BgTwf_ng_TOGJkZjM4ZTUtNGY5My00ZThhLTllNDQtYzJjMWJiMzJhZjNj [formerly /bz/th3.htm]]<br />
<br />
: One reader said "These docs are *brilliant* ! Exactly what I need to get an understanding of TH."<br />
<br />
<small>(Note: These documents are from [http://www.archive.org the Wayback machine] because the originals disappeared. They're public documents on Google docs, which shouldn't require logging in. However, if you're asked to sign in to view them, you're running into a known Google bug. You can fix it by browsing to [http://www.google.com Google], presumably gaining a cookie in the process.)</small><br />
<br />
<!--<br />
* Mark Snyder's Template Haskell chapter on the Software Generation and Configuration Report<br />
** http://nix.cs.uu.nl/dist/courses/sgc-report-unstable-latest/manual/chunk-chapter/templatehaskell.html<br />
--><br />
* A very short tutorial to understand the basics in 10 Minutes.<br />
** http://www.hyperedsoftware.com/blog/entries/first-stab-th.html<br />
<br />
* GHC Template Haskell documentation<br />
** http://www.haskell.org/ghc/docs/7.0.2/html/users_guide/template-haskell.html<br />
<br />
* Papers about Template Haskell<br />
<br />
** Template metaprogramming for Haskell, by Tim Sheard and Simon Peyton Jones, Oct 2002. [[http://www.haskell.org/wikiupload/c/ca/Meta-haskell.ps ps]]<br />
** Template Haskell: A Report From The Field, by Ian Lynagh, May 2003. [[http://www.haskell.org/wikiupload/2/24/Template_Haskell-A_Report_From_The_Field.ps ps]]<br />
** Unrolling and Simplifying Expressions with Template Haskell, by Ian Lynagh, December 2002. [[http://www.haskell.org/wikiupload/e/ed/Template-Haskell-Utils.ps ps]]<br />
** Automatic skeletons in Template Haskell, by Kevin Hammond, Jost Berthold and Rita Loogen, June 2003. [[http://www.haskell.org/wikiupload/6/69/AutoSkelPPL03.pdf pdf]]<br />
** Optimising Embedded DSLs using Template Haskell, by Sean Seefried, Manuel Chakravarty, Gabriele Keller, March 2004. [[http://www.haskell.org/wikiupload/b/b5/Seefried04th-pan.pdf pdf]]<br />
** Typing Template Haskell: Soft Types, by Ian Lynagh, August 2004. [[http://www.haskell.org/wikiupload/7/72/Typing_Template_Haskell_Soft_Types.ps ps]]<br />
<br />
= Other useful resources =<br />
<br />
* (2011) [https://github.com/leonidas/codeblog/blob/master/2011/2011-12-27-template-haskell.md Basic Tutorial of Template Haskell] <br />
<br />
* (2011) Greg Weber's [http://www.yesodweb.com/blog/2011/10/code-generation-conversation blog post on Template Haskell and quasi-quoting] in the context of Yesod.<br />
<br />
* (2012) Mike Ledger's [http://quasimal.com/posts/2012-05-25-quasitext-and-quasiquoting.html tutorial on TemplateHaskell and QuasiQuotation] for making an interpolated text QuasiQuoter.<br />
<br />
<!-- * [http://www.haskell.org/th/ The old Template Haskell web page]. Would someone feel like moving this material into the HaskellWiki? <br />
--><br />
<!-- * Old and probably not too useful for most but maybe... http://www.cse.unsw.edu.au/~chak/haskell/ghc/comm/exts/th.html <br />
--><br />
* [http://www.cs.ox.ac.uk/people/ian.lynagh/Fraskell/ Fraskell documentation] & explanation of how Template Haskell is used to vastly speed it up.<br />
<br />
* [[Quasiquotation]]<br />
<br />
Feel free to update our Wikipedia entry http://en.wikipedia.org/wiki/Template_Haskell<br />
<br />
= Projects =<br />
<br />
What are you doing/planning to do/have done with Template Haskell?<br />
<br />
* The [http://www.ict.kth.se/org/ict/ecs/sam/projects/forsyde/www ForSyDe methodology] is currently implemented as a Haskell-based DSL which makes extensive use of Template Haskell.<br />
<br />
* I have written a primitive (untyped) binding to the Objective-C runtime system on Mac OS X. It needs just TH, no "stub files" are created, no separate utilities are required. Initial snapshot is at http://www.kfunigraz.ac.at/imawww/thaller/wolfgang/HOC020103.tar.bz2 -- WolfgangThaller<br />
<br />
* I am writing Template Greencard - a reimplementation of GreenCard using TH. Many bits work out really nicely. A few bits didn't work so nicely - once I get some time to think, I'll try to persuade the TH folk to make some changes to fix some of these. -- AlastairReid<br />
<br />
* I'm writing Hacanon - a framework for automatic generation of C++ bindings. Read "automated Template Greencard for C++" (-: Darcs repo: http://www.ScannedInAvian.org/repos/hacanon - You'll need gccxml (http://www.gccxml.org/) to compile the examples. - 27 Dec Lemmih.<br />
<br />
* Following other FFI tools developers, I see some future for Template HSFFIG, especially when it comes to autogenerate peek and poke methods for structures defined in C; may be useful for implementation of certain network protocols such as X11 where layout of messages is provided as C structure/union declaration. - 16 Dec 2005 DimitryGolubovsky<br />
<br />
* I am using Template Haskell as a mechanism to get parsed, typechecked code into an Ajax based Haskell Equational Reasoning tool [[Haskell Equational Reasoning Assistant]], as well as simplify the specification of equational relationships between pieces of code. There was a quicktime movie of the tool being used on http://www.gill-warbington.com/home/andy/share/hera1.html - AndyGill <br />
<br />
* I am working on functional metaprogramming techniques to enhance programming reliability and productivity, by reusing much of the existing compiler technology. Template Haskell is especially interesting for me because it permits to check size information of structures by the compiler, provided this information is available at compile time. This approach is especially appropriate for hardware designs, where the structures are fixed before the circuit starts operating. See our metaprogramming web page at http://www.infosun.fmi.uni-passau.de/cl/metaprog/ -- ChristophHerrmann(http://www.cs.st-and.ac.uk/~ch)<br />
<br />
* I am using Template Haskell to do type safe database access. I initially [http://www.nabble.com/Using-Template-Haskell-to-make-type-safe-database-access-td17027286.html proposed this on haskell-cafe]. I connect to the database at compile-time and let the database do SQL parsing and type inference. The result from parsing and type inference is used to build a type safe database query which can executed at run-time. [[MetaHDBC | You can find the project page here]] -- [mailto:mads_lindstroem@yahoo.dk Mads Lindstrøm]<br />
<br />
= Utilities =<br />
<br />
Helper functions, debugging functions, or more involved code e.g. a monadic fold algebra for TH.Syntax.<br />
<br />
* http://www.haskell.org/pipermail/template-haskell/2003-September/000176.html<br />
<br />
= Known Bugs =<br />
<br />
Take a look at the [http://hackage.haskell.org/trac/ghc/query?status=new&status=assigned&status=reopened&component=Template+Haskell&order=priority open bugs against Template Haskell] on the GHC bug tracker.<br />
<br />
= Wish list =<br />
<br />
Things that Ian Lynagh (Igloo) mentioned in his paper ''Template Haskell: A Report From The Field'' in May 2003 (available [http://www.haskell.org/wikiupload/2/24/Template_Haskell-A_Report_From_The_Field.ps here]), by section:<br />
<br />
* Section 2 (curses)<br />
** The ability to splice names (into "foreign import" declarations, in particular)<br />
** The ability to add things to the export list from a splice(?)<br />
** The ability to use things defined at the toplevel of a module from splices in that same module (would require multi-stage compilation, as opposed to the current approach of expanding splices during typechecking)<br />
<br />
* Section 3 (deriving instances of classes)<br />
** <strike>First-class reification</strike> (the <hask>reify</hask> function)<br />
** A way to discover whether a data constructor was defined infix or prefix (which is necessary to derive instances for <hask>Read</hask> and <hask>Show</hask> as outlined in [http://www.haskell.org/onlinereport/derived.html The Haskell 98 Report: Specification of Derived Instances]) (if there is a way, [http://community.haskell.org/~ndm/derive/ Derive] seems ignorant of it)<br />
** Type/context splicing (in <hask>instance</hask> headers in particular)<br />
<br />
* Section 4 (printf)<br />
** He says something to the effect that a pattern-matching form of the quotation brackets would be cool if it was expressive enough to be useful, but that this would be hard. (Don't expect this anytime soon.)<br />
<br />
* Section 5 (fraskell)<br />
** Type information for quoted code (so that simplification can be done safely even with overloaded operations, like, oh, <hask>(+)</hask>)<br />
<br />
* Section 6 (pan)<br />
** Type info again, and strictness info too (this one seems a bit pie-in-the-sky...)<br />
<br />
(Please leave the implemented ones here, but crossed off.)<br />
<br />
Any other features that may be nice, and TH projects you'd want to see.<br />
<br />
* A TH tutorial (mainly a distillation and update of ''Template Meta-programming in Haskell'' at this point)<br />
* <strike>Write Haddock documentation for the Template Haskell library (http://hackage.haskell.org/trac/ghc/ticket/1576).</strike><br />
* Make `reify` on a class return a list of the instances of that class (http://www.haskell.org/pipermail/template-haskell/2005-December/000503.html). (See also [http://hackage.haskell.org/trac/ghc/ticket/1577 GHC ticket #1577].)<br />
* A set of simple examples on this wiki page<br />
* A TH T-shirt with new logo to wear at conferences<br />
* (Long-term) Unify Language.Haskell.Syntax with Language.Haskell.TH.Syntax so there's just one way to do things (http://hackage.haskell.org/package/haskell-src-meta does a one-way translation, for haskell-src-exts)<br />
<br />
---------------<br />
<br />
= Tips and Tricks =<br />
<br />
== What to do when you can't splice that there ==<br />
<br />
When you try to splice something into the middle of a template and find that you just can't, instead of getting frustrated about it, why not use the template to see what it would look like in longhand? <br />
<br />
First, an excerpt from a module of my own. I, by the way, am SamB.<br />
<haskell><br />
{-# OPTIONS_GHC -fglasgow-exts -fth #-}<br />
<br />
module MMixMemory where<br />
<br />
import Data.Int<br />
import Data.Word<br />
<br />
class (Integral int, Integral word)<br />
=> SignConversion int word | int -> word, word -> int where<br />
<br />
toSigned :: word -> int<br />
toSigned = fromIntegral<br />
toUnsigned :: int -> word<br />
toUnsigned = fromIntegral<br />
<br />
</haskell><br />
<br />
Say I want to find out what I need to do to splice in the types for an instance declaration for the SignConversion class, so that I can declare instances for Int8 with Word8 through Int64 with Word64. So, I start up good-ol' GHCi and do the following:<br />
<br />
<haskell><br />
$ ghci -fth -fglasgow-exts<br />
Prelude> :l MMixMemory<br />
*MMixMemory> :m +Language.Haskell.TH.Syntax<br />
*MMixMemory Language.Haskell.TH.Syntax> runQ [d| instance SignConversion Int Word where |] >>= print<br />
[InstanceD [] (AppT (AppT (ConT MMixMemory.SignConversion) (ConT GHC.Base.Int)) (ConT GHC.Word.Word)) []]<br />
</haskell><br />
<br />
== What can <tt>reify</tt> see? ==<br />
<br />
When you use <tt>reify</tt> to give you information about a <tt>Name</tt>, GHC will tell you what it knows. But sometimes it doesn't know stuff. In particular<br />
<br />
* '''Imported things'''. When you reify an imported function, type constructor, class, etc, from (say) module M, GHC runs off to the interface file <tt>M.hi</tt> in which it deposited all the info it learned when compiling M. However, if you compiled M without optimisation (ie <tt>-O0</tt>, the default), and without <tt>-XTemplateHaskell</tt>, GHC tries to put as little info in the interface file as possible. (This is a possibly-misguided attempt to keep interface files small.) In particular, it may dump only the name and kind of a data type into <tt>M.hi</tt>, but not its constructors.<br />
: Under these circumstances you may reify a data type but get back no information about its data constructors or fields. Solution: compile M with <br />
:* <tt>-O</tt>, or<br />
:* <tt>-fno-omit-interface-pragmas</tt> (implied by -O), or<br />
:* <tt>-XTemplateHaskell</tt>.<br />
<br />
* '''Function definitions'''. The <tt>VarI</tt> constructor of the <tt>Info</tt> type advertises that you might get back the source code for a function definition. In fact, GHC currently (7.4) <em>always</em> returns <tt>Nothing</tt> in this field. It's a bit awkward and no one has really needed it.<br />
<br />
== Why does <tt>runQ</tt> crash if I try to reify something? ==<br />
<br />
This program will fail with an error message when you run it:<br />
<haskell><br />
main = do info <- runQ (reify (mkName "Bool")) -- more hygenic is: (reify ''Bool)<br />
putStrLn (pprint info)<br />
</haskell><br />
Reason: <tt>reify</tt> consults the type environment, and that is not available at run-time. The type of <tt>reify</tt> is <br />
<haskell><br />
reify :: Quasi m => Q a -> m a<br />
</haskell><br />
The IO monad is a poor-man's instance of <tt>Quasi</tt>; it can allocate unique names and gather error messages, but it can't do <tt>reify</tt>. This error should really be caught statically.<br />
<br />
Instead, you can run the splice directly (ex. in ghci -XTemplateHaskell), as the following shows:<br />
<br />
<haskell><br />
GHCi> let tup = $(tupE $ take 4 $ cycle [ [| "hi" |] , [| 5 |] ])<br />
GHCi> :type tup<br />
tup :: ([Char], Integer, [Char], Integer)<br />
<br />
GHCi> tup<br />
("hi",5,"hi",5)<br />
<br />
GHCi> $(stringE . show =<< reify ''Int)<br />
"TyConI (DataD [] GHC.Types.Int [] [NormalC GHC.Types.I# [(NotStrict,ConT GHC.Prim.Int#)]] [])"<br />
</haskell><br />
<br />
Here's an [http://www.haskell.org/pipermail/glasgow-haskell-users/2006-August/010844.html email thread with more details].<br />
<br />
-----------------<br />
<br />
= Examples =<br />
== Tuples ==<br />
=== Select from a tuple ===<br />
<br />
An example to select an element from a tuple of arbitrary size. Taken from [http://research.microsoft.com/en-us/um/people/simonpj/papers/meta-haskell/meta-haskell.pdf this paper].<br />
<br />
Use like so:<br />
<br />
> $(sel 2 3) ('a','b','c')<br />
'b'<br />
> $(sel 3 4) ('a','b','c','d')<br />
'c'<br />
<br />
<br />
<haskell><br />
sel :: Int -> Int -> ExpQ<br />
sel i n = [| \x -> $(caseE [| x |] [alt]) |]<br />
where alt :: MatchQ<br />
alt = match pat (normalB rhs) []<br />
<br />
pat :: Pat<br />
pat = tupP (map varP as)<br />
<br />
rhs :: ExpQ<br />
rhs = varE(as !! (i -1)) -- !! is 0 based<br />
<br />
as :: [String]<br />
as = ["a" ++ show i | i <- [1..n] ]<br />
</haskell><br />
<br />
Alternately:<br />
<br />
<haskell><br />
sel' i n = lamE [pat] rhs<br />
where pat = tupP (map varP as)<br />
rhs = varE (as !! (i - 1))<br />
as = [ "a" ++ show j | j <- [1..n] ]<br />
</haskell><br />
<br />
=== Apply a function to the n'th element ===<br />
<br />
<haskell><br />
tmap i n = do<br />
f <- newName "f"<br />
as <- replicateM n (newName "a")<br />
lamE [varP f, tupP (map varP as)] $<br />
tupE [ if i == i'<br />
then [| $(varE f) $a |]<br />
else a<br />
| (a,i') <- map varE as `zip` [1..] ]<br />
</haskell><br />
<br />
Then tmap can be called as:<br />
<br />
> $(tmap 3 4) (+ 1) (1,2,3,4)<br />
(1,2,4,4)<br />
<br />
=== Convert the first n elements of a list to a tuple ===<br />
<br />
This example creates a tuple by extracting elements from a list. Taken from<br />
[http://www.xoltar.org/2003/aug/13/templateHaskellTupleSample.html www.xoltar.org]<br />
<br />
Use like so:<br />
<br />
> $(tuple 3) [1,2,3,4,5]<br />
(1,2,3)<br />
> $(tuple 2) [1,2]<br />
(1,2)<br />
<br />
<haskell><br />
tuple :: Int -> ExpQ<br />
tuple n = [|\list -> $(tupE (exprs [|list|])) |]<br />
where<br />
exprs list = [infixE (Just (list))<br />
(varE "!!")<br />
(Just (litE $ integerL (toInteger num)))<br />
| num <- [0..(n - 1)]]<br />
</haskell><br />
<br />
An alternative that has more informative errors (a failing pattern match failures give an exact location):<br />
<br />
<haskell><br />
tuple :: Int -> ExpQ<br />
tuple n = do<br />
ns <- replicateM n (newName "x")<br />
lamE [foldr (\x y -> conP '(:) [varP x,y]) wildP ns] (tupE $ map varE ns)<br />
</haskell><br />
<br />
=== Un-nest tuples ===<br />
Convert nested tuples like (a,(b,(c,()))) into (a,b,c) given the length to generate.<br />
<br />
<haskell><br />
unNest n = do<br />
vs <- replicateM n (newName "x")<br />
lamE [foldr (\a b -> tupP [varP a , b])<br />
(conP '() [])<br />
vs]<br />
(tupE (map varE vs))<br />
</haskell><br />
<br />
<br />
<br />
== [[Template Haskell/Marshall Data|Marshall a datatype to and from Dynamic]] ==<br />
This approach is an example of using template haskell to delay typechecking<br />
to be able to abstract out the repeated calls to fromDynamic:<br />
<br />
<haskell><br />
data T = T Int String Double<br />
<br />
toT :: [Dynamic] -> Maybe T<br />
toT [a,b,c] = do<br />
a' <- fromDynamic a<br />
b' <- fromDynamic b<br />
c' <- fromDynamic c<br />
return (T a' b' c')<br />
toT _ = Nothing<br />
</haskell><br />
<br />
== Printf ==<br />
Build it using a command similar to:<br />
<br />
ghc --make Main.hs -o main<br />
<br />
Main.hs:<br />
<br />
<haskell><br />
{-# LANGUAGE TemplateHaskell #-}<br />
<br />
-- Import our template "printf"<br />
import PrintF (printf)<br />
<br />
-- The splice operator $ takes the Haskell source code<br />
-- generated at compile time by "printf" and splices it into<br />
-- the argument of "putStrLn".<br />
main = do<br />
putStrLn $ $(printf "Hello %s %%x%% %d %%x%%") "World" 12<br />
</haskell><br />
<br />
PrintF.hs:<br />
<br />
<haskell><br />
{-# LANGUAGE TemplateHaskell #-}<br />
module PrintF where<br />
<br />
-- NB: printf needs to be in a separate module to the one where<br />
-- you intend to use it.<br />
<br />
-- Import some Template Haskell syntax<br />
import Language.Haskell.TH<br />
<br />
-- Possible string tokens: %d %s and literal strings<br />
data Format = D | S | L String<br />
deriving Show<br />
<br />
-- a poor man's tokenizer<br />
tokenize :: String -> [Format]<br />
tokenize [] = []<br />
tokenize ('%':c:rest) | c == 'd' = D : tokenize rest<br />
| c == 's' = S : tokenize rest<br />
tokenize (s:str) = L (s:p) : tokenize rest -- so we don't get stuck on weird '%'<br />
where (p,rest) = span (/= '%') str<br />
<br />
-- generate argument list for the function<br />
args :: [Format] -> [PatQ]<br />
args fmt = concatMap (\(f,n) -> case f of<br />
L _ -> []<br />
_ -> [varP n]) $ zip fmt names<br />
where names = [ mkName $ 'x' : show i | i <- [0..] ]<br />
<br />
-- generate body of the function<br />
body :: [Format] -> ExpQ<br />
body fmt = foldr (\ e e' -> infixApp e [| (++) |] e') (last exps) (init exps)<br />
where exps = [ case f of<br />
L s -> stringE s<br />
D -> appE [| show |] (varE n)<br />
S -> varE n<br />
| (f,n) <- zip fmt names ]<br />
names = [ mkName $ 'x' : show i | i <- [0..] ]<br />
<br />
-- glue the argument list and body together into a lambda<br />
-- this is what gets spliced into the haskell code at the call<br />
-- site of "printf"<br />
printf :: String -> Q Exp<br />
printf format = lamE (args fmt) (body fmt)<br />
where fmt = tokenize format<br />
</haskell><br />
<br />
== Handling Options with Templates ==<br />
A common idiom for treating a set of options, e.g. from GetOpt, is to define a datatype with all the flags and using a list over this datatype:<br />
<br />
<haskell><br />
data Options = B1 | B2 | V Integer<br />
<br />
options = [B1, V 3]<br />
</haskell><br />
<br />
While it's simple testing if a Boolean flag is set (simply use "elem"), it's harder to check if an option with an argument is set. It's even more tedious writing helper-functions to obtain the value from such an option since you have to explicitely "un-V" each. Here, Template Haskell can be (ab)used to reduce this a bit. The following example provides the module "OptionsTH" which can be reused regardless of the constructors in "Options". Let's start with showing how we'd like to be able to program. Notice that the resulting lists need some more treatment e.g. through "foldl".<br />
<br />
Options.hs:<br />
<br />
<haskell><br />
module Main where<br />
<br />
import OptionsTH<br />
import Language.Haskell.TH.Syntax<br />
<br />
data Options = B1 | B2 | V Int | S String deriving (Eq, Read, Show)<br />
<br />
options = [B1, V 3]<br />
<br />
main = do<br />
print foo -- test if B1 set: [True,False]<br />
print bar -- test if V present, w/o value: [False,True]<br />
print baz -- get value of V if available: [Nothing,Just 3]<br />
<br />
foo :: [Bool]<br />
-- Query constructor B1 which takes no arguments<br />
foo = map $(getopt (THNoArg (mkArg "B1" 0))) options<br />
<br />
bar :: [Bool]<br />
-- V is a unary constructor. Let mkArg generate the required<br />
-- wildcard-pattern "V _".<br />
bar = map $(getopt (THNoArg (mkArg "V" 1))) options<br />
<br />
-- Can't use a wildcard here!<br />
baz :: [(Maybe Int)]<br />
baz = map $(getopt (THArg (conP "V" [varP "x"]))) options<br />
</haskell><br />
<br />
OptionsTH.hs<br />
<br />
<haskell><br />
module OptionsTH where<br />
<br />
import Language.Haskell.TH.Syntax<br />
<br />
-- datatype for querying options:<br />
-- NoArg: Not interested in value (also applies to Boolean flags)<br />
-- Arg: Grep value of unary(!) constructor<br />
data Args = THNoArg Pat | THArg Pat<br />
<br />
getopt :: Args -> ExpQ<br />
getopt (THNoArg pat) = lamE [varP "y"] (caseE (varE "y") [cons0, cons1])<br />
where<br />
cons0 = match pat (normalB [| True |]) []<br />
cons1 = match wildP (normalB [| False |]) []<br />
<br />
-- bind "var" for later use!<br />
getopt (THArg pat@(ConP _ [VarP var])) = lamE [varP "y"] (caseE (varE "y") [cons0, cons1])<br />
where<br />
cons0 = match pat (normalB (appE [|Just|] (varE var))) []<br />
cons1 = match wildP (normalB [|Nothing|]) []<br />
<br />
mkArg :: String -> Int -> Pat<br />
mkArg k c = conP k (replicate c wildP)<br />
</haskell><br />
<br />
While the example might look contrived for the Boolean options which could have been tested much easier, it shows how both types of arguments can be treated in a similar way.<br />
<br />
=== Limitations ===<br />
<tt>getopt (THArg pat)</tt> is only able to treat unary constructors. See the pattern-binding: It matches exactly a single VarP.<br />
<br />
=== Improvements ===<br />
The following reduces things even a bit more, though I still don't know if I like it. It only works since <tt>c</tt> is either 0 or 1.<br />
<br />
<haskell><br />
mkArg k c = conP k (replicate c (varP "x"))<br />
<br />
baz = map $(getopt (THArg (mkArg "V" 1)))<br />
</haskell><br />
-- VolkerStolz<br />
<br />
== Generic constructor for records ==<br />
<br />
I have a large number of record types like this, of different length:<br />
<br />
<haskell><br />
data PGD = PGD {<br />
pgdXUnitBase :: !Word8,<br />
pgdYUnitBase :: !Word8,<br />
pgdXLUnitsperUnitBase :: !Word16<br />
}<br />
</haskell><br />
<br />
Currently I use GHC's Binary module to read them from files; it can handle<br />
types like <tt>(Word8, (Word8, Word16))</tt>, but there was no easy way to generate<br />
the correct amount of "uncurry" calls for automatically grabbing each element.<br />
<br />
With Template Haskell, the instance declarations are now written as such:<br />
<br />
<haskell><br />
instance Binary PGD where<br />
get bh = do a <- get bh ; return $ $(constrRecord PGD) a<br />
</haskell><br />
<br />
Here the trick lies in constrRecord, which is defined as:<br />
<br />
<haskell><br />
constrRecord x = reify exp where<br />
reify = \(Just r) -> appE r $ conE $ last args<br />
exp = foldl (dot) uncur $ replicate terms uncur<br />
terms = ((length args) `div` 2) - 2<br />
dot x y = (Just $ infixE x (varE ".") y)<br />
uncur = (Just [|uncurry|])<br />
args = words . show $ typeOf x<br />
</haskell><br />
<br />
-- AutrijusTang<br />
<br />
== zipWithN ==<br />
<br />
Here $(zipn 3) = zipWith3 etc.<br />
<br />
<haskell><br />
import Language.Haskell.TH; import Control.Applicative; import Control.Monad<br />
<br />
zipn n = do<br />
vs <- replicateM n (newName "vs")<br />
[| \f -><br />
$(lamE (map varP vs)<br />
[| getZipList $<br />
$(foldl<br />
(\a b -> [| $a <*> $b |])<br />
[| pure f |]<br />
(map (\v -> [| ZipList $(varE v) |]) vs))<br />
|])<br />
|]<br />
</haskell><br />
<br />
== 'generic' zipWith ==<br />
A generalization of zipWith to almost any data. Demonstrates the ability to do dynamic binding with TH splices (note 'dyn').<br />
<br />
<haskell><br />
zipCons :: Name -> Int -> [String] -> ExpQ<br />
zipCons tyName ways functions = do<br />
let countFields :: Con -> (Name,Int)<br />
countFields x = case x of<br />
NormalC n (length -> fields) -> (n, fields)<br />
RecC n (length -> fields) -> (n,fields)<br />
InfixC _ n _ -> (n,2)<br />
ForallC _ _ ct -> countFields ct<br />
<br />
TyConI (DataD _ _ _ [countFields -> (c,n)] _) <- reify tyName<br />
when (n /= length functions) $ fail "wrong number of functions named"<br />
vs <- replicateM ways $ replicateM n $ newName "x"<br />
lamE (map (conP c . map varP) vs) $<br />
foldl (\con (vs,f) -><br />
con `appE`<br />
foldl appE<br />
(dyn f)<br />
(map varE vs))<br />
(conE c)<br />
(transpose vs `zip` functions)<br />
</haskell><br />
<br />
This example uses whichever '+' is in scope when the expression is spliced:<br />
<br />
<haskell><br />
:type $(zipCons ''(,,,) 2 (replicate 4 "+"))<br />
<br />
$(zipCons ''(,,,) 2 (replicate 4 "+"))<br />
:: (Num t, Num t1, Num t2, Num t3) =><br />
(t, t1, t2, t3) -> (t, t1, t2, t3) -> (t, t1, t2, t3)<br />
</haskell><br />
<br />
==[[Template haskell/Instance deriving example|Instance deriving example]]==<br />
An example using a 'deriving function' to generate a method instance <br />
per constructor of a type. The deriving function provides the body of the<br />
method.<br />
<br />
Note that this example assumes that the functions of the class take a parameter that is the same type as instance is parameterized with. <br />
<br />
The message [http://www.haskell.org/pipermail/template-haskell/2006-August/000581.html email message] contains the full source ([http://www.iist.unu.edu/~vs/haskell/TH_render.hs extracted file]).<br />
<br />
== [[Quasiquotation|QuasiQuoters]] ==<br />
New in ghc-6.10 is -XQuasiQuotes, which allows one to extend GHC's syntax from library code. Quite a few examples are given in [http://hackage.haskell.org/package/haskell-src-meta haskell-src-meta].<br />
<br />
=== Similarity with splices ===<br />
<br />
Quasiquoters used in expression contexts (those using the ''quoteExp'') behave to a first approximation like regular TH splices:<br />
<br />
<haskell><br />
simpleQQ = QuasiQuoter { quoteExp = stringE } -- in another module<br />
<br />
[$simpleQQ| a b c d |] == $(quoteExp simpleQQ " a b c d ")<br />
</haskell><br />
<br />
[[Category:Language extensions]]</div>MikeLedgerhttps://wiki.haskell.org/index.php?title=Quasiquotation&diff=45789Quasiquotation2012-05-26T22:56:02Z<p>MikeLedger: add QuasiText and a tutorials section</p>
<hr />
<div>This is a tutorial for the quasiquoting facility described in<br />
[http://www.eecs.harvard.edu/~mainland/ghc-quasiquoting/mainland07quasiquoting.pdf Why It's Nice to be Quoted: Quasiquoting for Haskell].<br />
<br />
Quasiquoting allows programmers to use custom, domain-specific syntax to construct fragments of their program. Along with Haskell's existing support for domain specific languages, you are now free to use new syntactic forms for your EDSLs.<br />
<br />
More information on GHC's support for quasiquoting may be found:<br />
<br />
* [http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell.html#th-quasiquotation Using QQ with Template Haskell]<br />
* [http://www.eecs.harvard.edu/~mainland/ghc-quasiquoting/mainland07quasiquoting.pdf Why It's Nice to be Quoted: Quasiquoting for Haskell] :: PDF<br />
<br />
And a number of production examples:<br />
<br />
* The [[jmacro]] JavaScript generation library.<br />
* [http://hackage.haskell.org/package/interpolatedstring-qq Quasiquoter for Ruby-style interpolated strings]<br />
* [http://hackage.haskell.org/package/interpolatedstring-perl6 interpolatedstring-perl6 library: QuasiQuoter for Perl6-style multi-line interpolated strings with "q"]<br />
* [http://hackage.haskell.org/package/regexqq Quasiquoter for regular expressions]<br />
* [http://hackage.haskell.org/package/lighttpd-conf-qq A QuasiQuoter for lighttpd configuration files]<br />
* [http://hackage.haskell.org/package/haskell-src-meta haskell-src-meta contains quite a few QuasiQuoters]<br />
* [http://tanakh.github.com/Peggy/ Peggy: A Quasiquoter for Quasiquoters]<br />
* [http://hackage.haskell.org/package/QuasiText A QuasiQuoter for interpolated Text literals, with support for embedding Haskell expressions]<br />
<br />
Other QuasiQuotation/TemplateHaskell tutorials include:<br />
* [http://quasimal.com/posts/2012-05-25-quasitext-and-quasiquoting.html A look at QuasiQuotation]<br />
<br />
Note that the syntax for quasiquotation has changed since the paper was written: in GHC 7 one writes <hask>[expr|...|]</hask> instead of <hask>[:expr|...|]</hask>. GHC 6.12 uses <hask>[$expr|...|]</hask>. Quasiquotation appeared in GHC 6.9 and is enabled with the <code>QuasiQuotes</code> language option (<code>-XQuasiQuotes</code> on the command line or <hask>{-# LANGUAGE QuasiQuotes #-}</hask> in a source file).<br />
<br />
We show how to build a quasiquoter for a simple mathematical expression<br />
language. Although the example is small, it demonstrates all aspects of building<br />
a quasiquoter. We do not mean to suggest that one gains much from a quasiquoter<br />
for such a small language relative to using abstract syntax directly except from<br />
a pedagogical point of view---this is just a tutorial!<br />
<br />
The tutorial is runnable if its contents is placed in files as follows:<br />
<br />
Place the contents of the [[#Syntax]] and [[#Parsing]] sections in the file <code>Expr.hs</code><br />
with header<br />
<br />
<haskell><br />
{-# LANGUAGE DeriveDataTypeable #-}<br />
module Expr (Expr(..),<br />
BinOp(..),<br />
eval,<br />
parseExpr)<br />
where<br />
<br />
import Data.Generics<br />
import Text.ParserCombinators.Parsec<br />
</haskell><br />
<br />
Place the contents of the section [[#The Quasiquoter]] in a<br />
file <code>Expr/Quote.hs</code> with header<br />
<br />
<haskell><br />
module Expr.Quote (expr) where<br />
<br />
import Data.Generics<br />
import qualified Language.Haskell.TH as TH<br />
import Language.Haskell.TH.Quote<br />
<br />
import Expr<br />
</haskell><br />
<br />
= Syntax =<br />
<br />
Our simple expression language consists of integers, the standard operators<br />
+,x,*,/, and parenthesized expressions. We will write a single parser that takes<br />
concrete syntax for this language and transforms it to abstract syntax. Using<br />
the SYB approach to generic programming, we will then use this parser to produce<br />
expression and pattern quasiquoters. Our quasiquoter will allow us to write<br />
<hask>[expr|1 + 3|]</hask> directly in Haskell source code instead of the<br />
corresponding abstract syntax.<br />
<br />
An obvious datatype for the abstract syntax of this simple language is:<br />
<br />
<haskell><br />
data Expr = IntExpr Integer<br />
| BinopExpr (Integer -> Integer -> Integer) Expr Expr<br />
deriving(Show)<br />
</haskell><br />
<br />
Unfortunately, this won't do for our quasiquoter. First of all, the SYB<br />
technique we use cannot handle function types in a generic way, so the BinopExpr<br />
constructor must be modified. SYB also requires that we derive Typeable and<br />
Data, a trivial change. Finally, we want to support antiquoting for two<br />
syntactic categories, expressions and integers. With antiquoting support, we can<br />
write [expr|$x + $int:y|] where x and y are in-scope variables with types Expr<br />
and Integer, respectively. The final data types for our abstract syntax are:<br />
<br />
<haskell><br />
data Expr = IntExpr Integer<br />
| AntiIntExpr String<br />
| BinopExpr BinOp Expr Expr<br />
| AntiExpr String<br />
deriving(Show, Typeable, Data)<br />
<br />
data BinOp = AddOp<br />
| SubOp<br />
| MulOp<br />
| DivOp<br />
deriving(Show, Typeable, Data)<br />
</haskell><br />
<br />
An evaluator for our abstract syntax can be written as follows:<br />
<br />
<haskell><br />
eval :: Expr -> Integer<br />
eval (IntExpr n) = n<br />
eval (BinopExpr op x y) = (opToFun op) (eval x) (eval y)<br />
where<br />
opToFun AddOp = (+)<br />
opToFun SubOp = (-)<br />
opToFun MulOp = (*)<br />
opToFun DivOp = div<br />
</haskell><br />
<br />
= Parsing =<br />
<br />
We use Parsec to write a parser for our expression language. Note that we have<br />
(somewhat arbitrarily) chosen the syntax for antiquotaton to be as in the above<br />
example; a quasiquoter may choose whatever syntax she wishes.<br />
<br />
<haskell><br />
small = lower <|> char '_'<br />
large = upper<br />
idchar = small <|> large <|> digit <|> char '\''<br />
<br />
lexeme p = do{ x <- p; spaces; return x }<br />
symbol name = lexeme (string name)<br />
parens p = between (symbol "(") (symbol ")") p<br />
<br />
expr :: CharParser st Expr<br />
expr = term `chainl1` addop<br />
<br />
term :: CharParser st Expr<br />
term = factor `chainl1` mulop<br />
<br />
factor :: CharParser st Expr<br />
factor = parens expr <|> integer <|> try antiIntExpr <|> antiExpr<br />
<br />
mulop = do{ symbol "*"; return $ BinopExpr MulOp }<br />
<|> do{ symbol "/"; return $ BinopExpr DivOp }<br />
<br />
addop = do{ symbol "+"; return $ BinopExpr AddOp }<br />
<|> do{ symbol "-"; return $ BinopExpr SubOp }<br />
<br />
integer :: CharParser st Expr<br />
integer = lexeme $ do{ ds <- many1 digit ; return $ IntExpr (read ds) }<br />
<br />
ident :: CharParser s String<br />
ident = do{ c <- small; cs <- many idchar; return (c:cs) }<br />
<br />
antiIntExpr = lexeme $ do{ symbol "$int:"; id <- ident; return $ AntiIntExpr id }<br />
antiExpr = lexeme $ do{ symbol "$"; id <- ident; return $ AntiExpr id }<br />
</haskell><br />
<br />
The helper function parseExpr takes a source code position (consisting of a file<br />
name, line and column) and a string and returns a value of type Expr. This<br />
helper function also ensures that we can parse the whole string rather than just<br />
a prefix.<br />
<br />
<haskell><br />
parseExpr :: Monad m => (String, Int, Int) -> String -> m Expr<br />
parseExpr (file, line, col) s =<br />
case runParser p () "" s of<br />
Left err -> fail $ show err<br />
Right e -> return e<br />
where<br />
p = do pos <- getPosition<br />
setPosition $<br />
(flip setSourceName) file $<br />
(flip setSourceLine) line $<br />
(flip setSourceColumn) col $<br />
pos<br />
spaces<br />
e <- expr<br />
eof<br />
return e<br />
</haskell><br />
<br />
= The Quasiquoter =<br />
<br />
Remember, our quasiquoter allows us to write expression in our simple language,<br />
such as [expr|2 * 3|], directly in Haskell source code. This requires that the<br />
variable expr be in-scope when the quasiquote is encountered, and that it is<br />
bound to a value of type Language.Haskell.TH.Quote.QuasiQuoter, which contains<br />
an expression quoter and a pattern quoter. Note that expr must obey the same<br />
stage restrictions as Template Haskell; in particular, it may not be defined in<br />
the same module where it is used as a quasiquoter, but must be imported.<br />
<br />
Our expression and pattern quoters are quoteExprExp and quoteExprPat,<br />
respectively, so our quasiquoter expr is written as follows:<br />
<br />
<haskell><br />
quoteExprExp :: String -> TH.ExpQ<br />
quoteExprPat :: String -> TH.PatQ<br />
<br />
expr :: QuasiQuoter<br />
expr = QuasiQuoter quoteExprExp quoteExprPat<br />
</haskell><br />
<br />
Our quasiquoters re-use the parser we wrote in the previous section, parseExpr,<br />
and make use of the generic functions dataToExpQ and dataToPatQ (described in<br />
the Haskell Workshop paper). These functions, from the Language.Haskell.TH.Quote<br />
package, take a Haskell value and reflect it back into the language as Template<br />
Haskell abstract syntax. The catch is that we don't want to handle all values<br />
generically: antiquoted values must be handled specially. Consider the AntiExpr<br />
constructor; we don't want this constructor to be mapped to Template Haskell<br />
abstract syntax for the AntiExpr constructor, but to abstract syntax for the<br />
Haskell variable named by the constructor's argument. The extQ combinator allows<br />
us to do this nicely by defining a function antiExprExp that handles<br />
antiquotations.<br />
<br />
<haskell><br />
quoteExprExp s = do loc <- TH.location<br />
let pos = (TH.loc_filename loc,<br />
fst (TH.loc_start loc),<br />
snd (TH.loc_start loc))<br />
expr <- parseExpr pos s<br />
dataToExpQ (const Nothing `extQ` antiExprExp) expr<br />
<br />
antiExprExp :: Expr -> Maybe (TH.Q TH.Exp)<br />
antiExprExp (AntiIntExpr v) = Just $ TH.appE (TH.conE (TH.mkName "IntExpr"))<br />
(TH.varE (TH.mkName v))<br />
antiExprExp (AntiExpr v) = Just $ TH.varE (TH.mkName v)<br />
antiExprExp _ = Nothing<br />
</haskell><br />
<br />
The corresponding code for patterns is:<br />
<br />
<haskell><br />
quoteExprPat s = do loc <- TH.location<br />
let pos = (TH.loc_filename loc,<br />
fst (TH.loc_start loc),<br />
snd (TH.loc_start loc))<br />
expr <- parseExpr pos s<br />
dataToPatQ (const Nothing `extQ` antiExprPat) expr<br />
<br />
antiExprPat :: Expr -> Maybe (TH.Q TH.Pat)<br />
antiExprPat (AntiIntExpr v) = Just $ TH.conP (TH.mkName "IntExpr")<br />
[TH.varP (TH.mkName v)]<br />
antiExprPat (AntiExpr v) = Just $ TH.varP (TH.mkName v)<br />
antiExprPat _ = Nothing<br />
</haskell><br />
<br />
= Examples =<br />
<br />
We can now try out a few examples by invoking ghci as follows: <code>ghci -XQuasiQuotes Expr/Quote</code><br />
<br />
<pre><br />
> [expr|1 + 3 + 5|]<br />
BinopExpr AddOp (BinopExpr AddOp (IntExpr 1) (IntExpr 3)) (IntExpr 5) <br />
> eval [expr|1 + 3 + 5|]<br />
9 <br />
</pre><br />
<br />
Taking advantage of our quasiquoter, we can re-write our evaluator so it uses<br />
concrete syntax:<br />
<br />
<haskell><br />
eval' :: Expr -> Integer<br />
eval' [expr|$int:x|] = x<br />
eval' [expr|$x + $y|] = eval' x + eval' y<br />
eval' [expr|$x - $y|] = eval' x - eval' y<br />
eval' [expr|$x * $y|] = eval' x * eval' y<br />
eval' [expr|$x / $y|] = eval' x `div` eval' y<br />
</haskell><br />
<br />
Let's make sure it works as advertised:<br />
<br />
<pre><br />
> eval [expr|1 + 2 + 3|] == eval' [expr|1 + 2 + 3|]<br />
True <br />
> eval [expr|1 + 3 * 5|] == eval' [expr|1 + 3 * 5|]<br />
True <br />
</pre></div>MikeLedger