Hi,

I am playing with SPECIALIZE pragma:

module Todo where

{-# SPECIALIZE plusTwoRec :: [Int] -> [Int] #-}
plusTwoRec :: Num a => [a] -> [a]
plusTwoRec [] = []
plusTwoRec (x:xs) = x+2:plusTwoRec xs

plusTwoRec' :: Num a => [a] -> [a]
plusTwoRec' [] = []
plusTwoRec' (x:xs) = x+2:plusTwoRec' xs

And wanted to benchmark it with (in Main.hs):

import Todo
import Criterion.Main

aListOfInt :: [Int]
aListOfInt = [1..10000]

main :: IO ()
main = defaultMain
  [ bench "plusTwoRec" $ nf plusTwoRec aListOfInt
  , bench "plusTwoRec'" $  nf plusTwoRec' aListOfInt
  ]

Sadly, the rule of specialization of plusTwoRec does not fire in Main.hs (I compiled with: ghc Main.hs -O -dynamic -ddump-rule-firings (the -dynamic part is due to my ArchLinux installaltion)).

The result is:

[1 of 2] Compiling Todo             ( Todo.hs, Todo.o )
Rule fired: Class op + (BUILTIN)
Rule fired: Class op fromInteger (BUILTIN)
Rule fired: integerToInt (BUILTIN)
Rule fired: SPEC plusTwoRec (Todo)
[2 of 2] Compiling Main             ( Main.hs, Main.o )
Rule fired: Class op enumFromTo (BUILTIN)
Rule fired: unpack (GHC.Base)
Rule fired: unpack (GHC.Base)
Rule fired: eftIntList (GHC.Enum)
Rule fired: unpack-list (GHC.Base)
Rule fired: unpack-list (GHC.Base)
Linking Main ...

I have inspected a bit the code produced after the simplifications passes (with -ddump-simpl) and here is the suspicious part:

plusTwoRec :: forall a. Num a => [a] -> [a]
[GblId,
 Arity=2,
 Caf=NoCafRefs,
 Str=<L,U(C(C1(U)),A,A,A,A,A,C(U))><S,1*U>,
 Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
         WorkFree=True, Expandable=True,
         Guidance=ALWAYS_IF(arity=0,unsat_ok=True,boring_ok=True)}]
plusTwoRec = plusTwoRec'

I believe that plusTwoRec is inlined before the specialization has a chance to fire, but I am not sure at all !

Separating the two functions definitions in two different files works.

So I don't know if this is a GHC bug, myself that does not read the right part of the GHC manual, if it is only a lack of documentation, or anything else.