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(Let IS recursive)
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=== Let IS recursive ===
=== Let IS recursive ===
Most of the time the <haskell>let</haskell> in Core is non recursive, for example:
Most of the time the <code>let</code> in Core is non recursive, for example:
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where the <haskell>let</haskell> binding ''is'' recursive.
where the <haskell>let</haskell> binding ''is'' recursive.
== See Also ==
== See Also ==

Revision as of 18:01, 7 November 2006

Part of Yhc


Yhc Core is a way of dumping and using the internal representation of Yhc in an external project.


1 Haddock Documentation

2 Overview

Yhc.Core is a simple core Haskell-like language, feature case statements, let statements, top level lambda's and data values. Much of the syntactic sugar present in Haskell has gone (list comprehensions, typeclasses, overloaded names, nested lambdas)

The strengths of Yhc.Core are:

  • Simple representation of Haskell
  • Relatively simple to relate Core to original Haskell
  • Source locations are preseved
  • Minimal name mangling
  • Few syntactic forms

The weaknesses are:

  • Yhc cannot compile Yhc Core files, format is write only (fix is being worked on)
  • Types are not present (hard to fix, a lot of work)

The intended users:

  • Analysis tools
  • Compiler backends

3 Generating Core

Yhc Core files are stored as .ycr files in a binary format. They can be generated by passing the -core flag to the compiler:

yhc -corep Main.hs

If Core files are required for the standard libraries they can be built using scons core=1. To view the Core output, yhc Main -showcore, with the output going to the screen in a pretty printed format. To view an existing Core file, yhc -viewcore File.ycr.

4 The Core output

For an example of the Core output, taking the following program:

head2 (x:xs) = x
map2 f [] = []
map2 f (x:xs) = f x : map2 f xs
test x = map2 head2 x


Sample.head2 v220 =
    case v220 of
        (:) v221 v222 -> v221
        _ -> Prelude.error Sample._LAMBDA228
Sample._LAMBDA228 =
    "Sample: Pattern match failure in function at 9:1-9:15."
Sample.map2 v223 v224 =
    case v224 of
        [] -> []
        (:) v225 v226 -> (:) (v223 v225) (Sample.map2 v223 v226)
Sample.test v227 = Sample.map2 Sample.head2 v227

Note that all names have been fully qualified, there are no infix operators, all pattern matches have been converted to cases.

5 Some little samples

5.1 Hello World, for Core

The following snippet loads a Core file and prints it out.

import Yhc.Core
showFile x :: FilePath -> String
showFile x = liftM show (loadCore x)

5.2 The linker

Since all Core programs are fully qualified, linking is really easy:

import Yhc.Core
linker :: [Core] -> Core
linker xs = foldr f (Core "" [] [] []) xs
   where f (Core _ _ x1 x2) (Core _ _ y1 y2) = Core "" [] (x1++y1) (x2++y2)

Here the linker ignores the import statements and the module name, of course you can do linking driven by the import statements easily.


How do you change all strings and characters into uppercase?

import Yhc.Core
shout :: Core -> Core
shout core = mapUnderCore f core
        f (CoreChr x) = CoreChr (toUpper x)
        f (CoreStr x) = CoreStr (map toUpper x)
        f x = x

5.4 The 42 counter

How many of your functions contain the literal 42? How many times does it occur per function?

import Yhc.Core
main x = putStr $ unlines [name ++ ": " ++ show count | (name,count) <- count42 x]
count42 :: Core -> [(String,Int)]
count42 core = [(name, n) | func@(Func name _ _) <- coreFuncs core,
                            let n = length $ filter is42 $ allCore func, n /= 0]
is42 :: CoreExpr -> Bool
is42 (CoreInt 42) = True
is42 (CoreInteger 42) = True
is42 _ = False

6 Invariants

There are many invariants that hold in a Yhc Core program, and some that usually hold but aren't actually true. This secion lists both types.

6.1 Let IS recursive

Most of the time the let in Core is non recursive, for example:

main = let f True = False
           f False = f True
       in print $ f False


Main.main =
      (Prelude.print Prelude.Prelude.Show.Prelude.Bool)
      (Main.Main.Prelude.195.f Prelude.False)
Main.Main.Prelude.195.f v205 =
    case v205 of
        Prelude.True -> Prelude.False
        Prelude.False -> Main.Main.Prelude.195.f Prelude.True
Note how the recursive
has been expanded out.

However, there are a few places this isn't true - Prelude.repeat and Prelude.cycle are defined as:

repeat x = xs where xs = x : xs

And this desugars to:

Main.rep v212 =
    let Main.Main.Prelude.196.xs =
            (Prelude.:) v212 Main.Main.Prelude.196.xs
    in Main.Main.Prelude.196.xs
where the
binding is recursive.

7 See Also

8 Users

  • Catch - Case Totality Checker for Haskell by Neil Mitchell. This takes Core as an input.
  • Dr Haskell - give hints to beginners about useful Haskell functions/idioms.