Revision as of 14:18, 22 September 2007
|Part of Yhc|
Yhc Core is a way of dumping and using the internal representation of Yhc in an external project.
1 Haddock documentation
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 --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.
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 :: FilePath -> IO () showFile x = loadCore x >>= print
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
5.3 SHOUT AT EVERYONE
How do you change all strings and characters into uppercase?
import Yhc.Core shout :: Core -> Core shout core = mapUnderCore f core where 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
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 Primitives are saturated
Are all primitives called saturated? I don't know...
6.2 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.$) (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
let binding is recursive.
6.3 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:
gas 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
xis to be evaluated, and evaluate
x: this preserves laziness