[Haskell-cafe] Re: Monad explanation

Daryoush Mehrtash dmehrtash at gmail.com
Thu Feb 5 05:23:45 EST 2009


Can someone compare/contrast functions and computation?

thanks

daryoush

On Thu, Feb 5, 2009 at 1:38 AM, Benjamin L. Russell
<DekuDekuplex at yahoo.com>wrote:

> On Wed, 4 Feb 2009 21:43:04 -0800, Max Rabkin <max.rabkin at gmail.com>
> wrote:
>
> >On Wed, Feb 4, 2009 at 9:38 PM, Benjamin L. Russell
> ><DekuDekuplex at yahoo.com> wrote:
> >> Is it possible to write a self-referential function in Haskell that
> >> modifies itself?
> >
> >Is it possible to write *any* kind of function in Haskell that
> >modifies *anything*?
>
> While trying to research this issue, I came across a relevant archived
> thread in Haskell-Cafe, entitled "[Haskell-cafe] haskell and
> reflection," started by Greg Meredith, dated "Tue, 11 Sep 2007
> 07:09:22 -0700" (see
> http://www.mail-archive.com/[email protected]/msg29882.html),
> which at first had me worried.  Specifically, Greg wrote as follows:
>
> >Am i wrong in my assessment that the vast majority of reflective machinery
> >is missing from Haskell? Specifically,
> >
> >   - there is no runtime representation of type available for
> >   programmatic representation
> >   - there is no runtime representation of the type-inferencing or
> >   checking machinery
> >   - there is no runtime representation of the evaluation machinery
> >   - there is no runtime representation of the lexical or parsing
> >   machinery
>
> However, I was somewhat relieved to find that Haskell apparently does
> support, in at least one GHC extension to Haskell, a limited form of
> reflection.  In the same thread, Reinier Lamers, in his response,
> dated "Tue, 11 Sep 2007 13:08:38 -0700" (see
> http://www.mail-archive.com/[email protected]/msg29898.html),
> wrote as follows:
>
> >Op 11-sep-2007, om 18:43 heeft Greg Meredith het volgende geschreven:
> >
> >[...]
> >
> >Template Haskell [1] is a system that lets you write programs that get
> >executed at *compile time*, and that produce parts of the Haskell program
> >to be compiled by manipulating a representation of the program as
> structured
> >data. It's a form of reflection restricted to compile time, if you'd ask
> me.
> >
> >[...]
> >
> >[1] http://www.haskell.org/haskellwiki/Template_Haskell
>
> According to the site referenced by the above-mentioned link,
>
> >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.
>
> This site is even referenced on the site for "The Meta-Programming
> Project" (see
> http://www.infosun.fim.uni-passau.de/cl/metaprog/index.php3), as
> follows:
>
> >Languages which we are using for meta-programming
> >
> >    * Template Haskell, permits analysis of object programs
>
> Additionally, Don Stewart, in an earlier post in the same thread,
> dated "Thu, 13 Sep 2007 11:36:11 -0700" (see
> http://www.mail-archive.com/[email protected]/msg30009.html),
> wrote as follows:
>
> >lgreg.meredith:
> >>    Haskellians,
> >>
> >>    Am i wrong in my assessment that the vast majority of reflective
> machinery
> >>    is missing from Haskell? Specifically,
> >>
> >>      * there is no runtime representation of type available for
> programmatic
> >>        representation
> >
> >>      * there is no runtime representation of the type-inferencing or
> checking
> >>        machinery
> >
> >>      * there is no runtime representation of the evaluation machinery
> >
> >>      * there is no runtime representation of the lexical or parsing
> machinery
> >
> >So there is library support for all of this, in various forms:
> >
> >    * lexer, parser, type checker, evaluator:
> >            ghc-api
> >            hs-plugins
> >
> >    * lexer, parser, pretty printer
> >            many parser libs (Language.Haskell, Template Haskell)
> >
> >    * runtime type representation
> >            Data.Typeable
> >
> >    * reflecting code to data:
> >            Data.Generics
>
> As a sidenote, I discovered an interesting post and associated paper
> by Lauri Alanko, who in a post in the same thread, dated "Tue, 11 Sep
> 2007 10:12:09 -0700" (see
> http://www.mail-archive.com/[email protected]/msg29895.html),
> responded that while both structural and procedural reflection are
> possible in Haskell, because of static typing, type safety is
> nevertheless an issue:
>
> >On Tue, Sep 11, 2007 at 07:33:54AM -0700, Greg Meredith wrote:
> >> Our analysis suggested the following breakdown
> >>
> >>    - Structural reflection -- all data used in the evaluation of
> programs
> >>    has a programmatic representation
> >>    - Procedural reflection -- all execution machinery used in the
> >>    evaluation of programs has a programmatic representation
> >>
> >>[...]
> >
> >As for Haskell, there are various tools for both (at least
> >Data.Typeable and hs-plugins), but neither are truly type-safe: it is
> >possible to write code that uses these libraries and type-checks, yet
> >crashes. Static typing makes reflection very difficult to support
> >safely.
>
> >You might be interested in my MS thesis, where I explored these issues
> >in some more length:
> >http://www.cs.helsinki.fi/u/lealanko/alanko04types.pdf
>
> This thesis is entitled "Types and Reflection" (see the
> above-mentioned URL), and summarizes the problem as follows (see p.
> 1):
>
> >This thesis is about reflection in typed programming languages. The
> central
> >idea of this work, the thesis proper, can be summarized in three points:
> >. Typed reflection is a good thing.
> >. It has not yet been done properly.
> >. It is nevertheless possible.
>
> The basic problem with reflection in a statically typed language seems
> to be that "reflection wreaks havoc to the basic assumptions that type
> systems are based on" (see p. 11).  Specifically, he explains as
> follows (see p. 38):
>
> >[A] type system verifies or infers universal properties about all
> >possible executions of a program by making a context-sensitive syntactic
> analysis of the
> >program's structure. Usually this is possible because the computational
> rules for executing
> >the program are simple, tractable and most importantly syntax-directed:
> the evaluation
> >of each expression proceeds in a predictable fashion that is determined
> mostly by its syntactic
> >form and only to a limited extent by run-time state. For each expression
> there are
> >certain invariants that always hold for its evaluation, and these
> invariants can be expressed
> >in the expression's type.
> >
> >In practice this means that we can prove the soundness of a type system
> with structural
> >induction over the terms, using the dynamic semantics of the language to
> show that the
> >evaluation of each expression will yield a well-typed result.
> >
> >In the presence of reflective operations this mode of reasoning no longer
> works. The
> >reason is that the effects of all primitive expressions on computational
> structures are no
> >longer simple and tractable: an absorption operation turns an arbitrary
> run-time value
> >into a computational structure, and there are therefore no simple
> invariants about the
> >operation's behavior.
> >
> >This is of course an inherent property of reflection. Its idea, after all,
> is to allow the
> >program more freedom at run-time instead of completely predefining its
> behavior when
> >the program is written. Yet it is still a rarely used feature and it seems
> unreasonable that it
> >should make static typing completely unattainable. Thankfully, this proves
> not to be the
> >case.
>
> In Greg Meredith's solution, he writes as follows (see p. 42):
>
> >[T]here is a tradeoff to be made between static safety and dynamic
> flexibility.
> >It turns out that it is possible to do quite sophisticated run-time code
> manipulation while
> >still retaining fully static safety guarantees both of the original code
> and the generated
> >code.
>
> In Section 8.3: "Dynamics in Haskell," Meredith elucidates as follows
> (see p. 62):
>
> >The language Haskell [H98] has a powerful type system, but the standard
> version of the
> >language has no support for run-time type operations. However, there is a
> dynamics
> >library that has for a long time been included in many implementations
> [LPJ03, Section
> >8]. The library provides a datatype Dynamic and some machinery for
> injecting values
> >of different types into it and for extracting them out from it.
> Unfortunately, the user
> >of the library is required to obey certain programming conventions, and if
> they are not
> >followed, it is very easy to write code that breaks type safety. Moreover,
> the library
> >provides only limited support for dynamics. For instance, it is not
> possible to inject
> >polymorphic functions into a Dynamic.
> >
> >More recently, Cheney and Hinze [CH02a] and Baars and Swierstra [BS02]
> have developed
> >more flexible and type-safe frameworks for representing type information
> at runtime
> >in Haskell. In fact, dynamic values are only one of the possible
> applications of the
> >frameworks.
>
> Cheney and Hinze's system is then described in more detail.  Meredith
> then concludes as follows (see p. 63):
>
> >The above system is quite remarkable. The type Rep [tau]   bridges the gap
> between static and
> >dynamic type information while remaining completely type-safe even
> internally. Indeed,
> >the type representations are very much like reified types. They can be
> synthesized and
> >analyzed, and the isomorphisms allow a kind of absorption: conversion from
> run-time to
> >compile-time type information.
>
> Some problems relating to the need for programmer cooperation, the
> treatment of new named datatypes, and lack of full reflexivity are
> then described.  Nevertheless, Meredith summarizes as follows (see p.
> 64):
>
> >Nevertheless, these "lightweight dynamics" are surprisingly powerful,
> considering that
> >they can be implemented in a statically typed language without extending
> the type system.
> >For example, Baars and Swierstra demonstrate how to write a type-safe eval
> function for a
> >simple language so that all type checking is done before the code is
> evaluated. Although
> >this is not full reflection due to the simplicity of the object language,
> the result is still
> >encouraging. For one thing, at least now a type can be pretty-printed even
> without the
> >presence of a value of that type.
>
> Apparently, there is hope for reflection in Haskell with such
> libraries.
>
> What would be especially interesting would be a specific example of a
> type-safe Haskell program that could modify itself at runtime.  If
> anyone has any examples to cite, I would be very interested in reading
> about them in this thread.
>
> -- Benjamin L. Russell
> --
> Benjamin L. Russell  /   DekuDekuplex at Yahoo dot com
> http://dekudekuplex.wordpress.com/
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