Bad code with type families
{-# LANGUAGE TypeFamilies, ScopedTypeVariables #-}
module Foo (foo) where
import Control.Monad.ST
import Data.STRef
class V v where
type M v :: * -> *
rd :: v -> Int -> M v Int
data Vec s = Vec (STRef s Int)
instance V (Vec s) where
type M (Vec s) = ST s
rd (Vec r) n = do
m <- readSTRef r
return (m+n)
foo :: forall s. Vec s -> Int -> ST s Int
foo v n = go n
where
go 0 = return 0
go n = do
m <- rd v n -- :: ST s Int
go m
ghc -O2 generates rather bad code for this (look at $wfoo). If I uncomment the type signature in the second to last line, the code becomes much better. It would be nice if this worked without the signature since I can't add it in the real code.
Trac metadata
Trac field | Value |
---|---|
Version | 6.9 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | Unknown |
Architecture | Unknown |