Yhc/API/Core

Part of Yhc

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 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

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 --core Main.hs

If you want all definitions from all libraries included in then:

yhc --linkcore Main.hs

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.

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 cases.

Some little samples

Hello World, for Core

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

import Yhc.Core

showFile :: FilePath -> IO ()
showFile x = loadCore x >>= print

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.

Note: The --linkcore option in Yhc does automatic linking

SHOUT AT EVERYONE

How do you change all strings and characters into uppercase?

import Yhc.Core

shout :: Core -> Core
shout core = transformCore f core
    where
        f (CoreChr x) = CoreChr (toUpper x)
        f (CoreStr x) = CoreStr (map toUpper x)
        f x = x

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 $ universeCore func, n /= 0]

is42 :: CoreExpr -> Bool
is42 (CoreInt 42) = True
is42 (CoreInteger 42) = True
is42 _ = False

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.

Primitives are saturated

Are all primitives called saturated? I don't know...

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

generates:

Main.main =
    (Prelude.$)
      (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 let has been expanded out.

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

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 let binding is recursive.

Oversaturation is possible in Core

Consider this Haskell code

let g = fst tup  
    x = g (5::Int) (6::Int)

This desugars to:

let Bug.Bug.Prelude.217.g = -- arity = 0
  let v285 = Prelude.Prelude.Num.Prelude.Integer
  in Prelude.fst (Bug.tup v285)
in let Bug.Bug.Prelude.218.x = Bug.Bug.Prelude.217.g 5 6 -- called with 2 arguments

There can be two solutions to this:

evaluate g as soon as it is ready (i. e. before application to 5 6): this may eliminate some laziness
take care of oversaturating arguments by storing them until x is to be evaluated, and evaluate g right before x: this preserves laziness

An example how the second solution can be implemented in Javascript, see this section of the ycr2js page.

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.
ycr2js - a Core to Javascript translator.