Yhc/API/Core

Part of Yhc (Download)

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

Haddock Documentation

• http://www.cs.york.ac.uk/fp/yhc/snapshot/docs/Yhc-Core.html

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 synactic 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

Generating Core

Yhc Core files are stored as .ycr files in a binary format. They can be generated by passing the -corep flag to the compiler, yhc -corep Main.hs. If Core files are required for the standard libraries these can be build using scons core=1. To view the Core output, yhc -core Main.hs, with the output going to the screen in a pretty printed format.

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

Generates:

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 case's.

Some little samples

Hello World, for Core

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

import Yhc.Core

showFile x :: FilePath -> String
showFile x = liftM show (loadCore x)

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.

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 [CoreInt 42 <- allCore func], n /= 0]

See Also

• GHC Core

Users

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