I allege that this is a Hugs bug! Yes, delayTrans is
monomorphic, so it can only be used at one type.
And indeed, it *is* used at one type, namely Bool.
We use the defaults when the situation is ambiguous;
here it isn't.
Perhaps I should add a clarifying para to the report?
Simon
> -----Original Message-----
> From: Jeffrey R. Lewis [mailto:[EMAIL PROTECTED]]
> Sent: Tuesday, November 10, 1998 5:12 PM
> To: [EMAIL PROTECTED]
> Subject: monomorphism wierdness
>
>
> With GHC 4.00, the enclosed program successully compiles.
> Hugs reports
> the following (expected) type error (line 11 is the one that contains
> `delayTrans memOut'):
>
> ERROR "qh.hs" (line 11): Type error in application
> *** expression : delayTrans memOut
> *** term : memOut
> *** type : [Maybe Bool]
> *** does not match : [Maybe Int]
>
> If I look at the .hi file GHC produces, I see:
>
> MuHawk.hi:57 delayTrans _:_ [PrelMaybe.Maybe PrelBase.Bool] ->
> [PrelMaybe.Maybe PrelBase.Bool] ;;
>
> The correct type for delayTrans should be `[Maybe Int]' because of the
> monomorphism restriction. It's as if the `default' for Num were
> suddenly `Bool'. If I add the following two signatures, everything
> types properly:
>
> nopTrans :: Num a => Maybe a
> delayTrans :: Num a => [Maybe a] -> [Maybe a]
>
> ------------------
>
> module MuHawk where
>
> delay = (:)
>
> nopTrans = Just 0
>
> delayTrans = delay nopTrans
>
> memOut :: [Maybe Bool]
> memOut = error "memOut"
> writeback = delayTrans memOut
>
> instance Num Bool where
> (+) = (||)
> (*) = (&&)
> negate = not
> fromInteger n = if even n then False else True
>
>
