Opened 4 years ago

Closed 4 years ago

Last modified 4 years ago

#10767 closed bug (fixed)

SPECIALIZE generates warning but works fine

Reported by: osa1 Owned by:
Priority: normal Milestone: 8.0.1
Component: Compiler Version: 7.10.2
Keywords: Cc:
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: None/Unknown Test Case: deSugar/should_compile/T10767
Blocked By: Blocking:
Related Tickets: Differential Rev(s):
Wiki Page:


I have this code:

{-# LANGUAGE ScopedTypeVariables, TypeFamilies #-}

module Main where

import Data.Proxy

class SpecList a where
    type List a :: *

    slCase      :: List a -> b -> (a -> List a -> b) -> b

data IntList
  = ILNil
  | ILCons {-# UNPACK #-} !Int IntList
  deriving (Show)

instance SpecList Int where
  type List Int = IntList

  slCase ILNil        n _  = n
  slCase (ILCons i t) _ c  = c i t

fromList :: [Int] -> IntList
fromList []      = ILNil
fromList (h : t) = ILCons h (fromList t)

lst1 :: IntList
lst1 = fromList [1..10]

{-# SPECIALIZE genLength :: Proxy Int -> List Int -> Int #-}
genLength :: forall a . SpecList a => Proxy a -> List a -> Int
genLength p lst = slCase lst 0 (\(_ :: a) tail -> 1 + genLength p tail)

main :: IO ()
main = print (genLength (Proxy :: Proxy Int) lst1)

When I compile it(no matter which optimization level is used), it prints this warning:

Main.hs:30:1: Warning:
    RULE left-hand side too complicated to desugar
      Optimised lhs: case cobox_a17r
                     of _ [Occ=Dead] { GHC.Types.Eq# cobox ->
                     genLength @ Int $dSpecList_a17q
      Orig lhs: case cobox_a17r of cobox_a17r { GHC.Types.Eq# cobox ->
                genLength @ Int $dSpecList_a17q

(I presume this is related with the rewrite rule generated to be able to replace calls to generic version to calls to specialized versions when possible.)

But this program compiles fine with -O2, i.e. genLength lst1 is replaced with specialized definition of genLength, without any dictionary passing.

So the warning is confusing for two reasons:

  • It's not related with the code, it seems like it's a problem with GHC-generated rewrite rule.
  • It still works fine.

Change History (4)

comment:1 Changed 4 years ago by Thomas Miedema <thomasmiedema@…>

In 1395185f/ghc:

Testsuite: add test for #10767

comment:2 Changed 4 years ago by thomie

Milestone: 8.0.1
Resolution: fixed
Status: newclosed
Test Case: deSugar/should_compile/T10767

Fixed in HEAD. I added a test.

comment:3 Changed 4 years ago by osa1

Interesting, I spent some time debugging this myself so I was wondering what fixed this. @thomie do you have any ideas?

comment:4 Changed 4 years ago by thomie

I have no idea what fixed it.

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