Custom Query

Reported by igloo, 6 years ago.

In a cmd or a cygwin shell, but not when SSHed into cygwin from a Linux machine, pressing enter in ghci causes this:

C:\ghc\ghc-6.8.1.20071125\bin>.\ghci
GHCi, version 6.8.1.20071125: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
Prelude>
user error (MultiByteToWideChar: The parameter is incorrect.
 (error code: 57))

C:\ghc\ghc-6.8.1.20071125\bin>

This doesn't happen in 6.8.1.

Reported by Michael D. Adams, 6 years ago.

When experimenting with Cmm code, I found the following bug/trap.

This Cmm code,

foo {
    bits8 x;
    x = 5;
    bits8[12] = x;
    R1 = x;
    jump bar;
}

causes the following error

/tmp/ghc6613_0/ghc6613_0.s: Assembler messages:

/tmp/ghc6613_0/ghc6613_0.s:7:0:
     Error: junk `naughty I386 byte register' after expression

The assembly that is produced is:

.text
        .align 4,0x90
.globl foo
foo:
        movl $5,%eax
        movb %al,12
        movb %al,very naughty I386 byte register
        jmp bar

.section .note.GNU-stack,"",@progbits
.ident "GHC 6.7.20070612"

Note the "very naughty I386 byte register" in there.

I am not sure whether this kind of Cmm can be generated from Haskell code.

Reported by atnnn, 14 months ago.

Consider this GHCi session:

> value (Proxy :: Proxy (Just True))
No instance for (Value (Maybe Bool) (Just Bool 'True) t0)
> :i Value
instance Value Bool 'True Bool
instance Value k a t => Value (Maybe k) (Just k a) (Maybe t)
> value (Proxy :: Proxy (Just True)) :: Maybe Bool
Just True

It would be helpful if GHC could use the instance (which seems suitable) or emit a better error message.

{-# LANGUAGE DataKinds, MultiParamTypeClasses, FunctionalDependencies,
             PolyKinds, UndecidableInstances, ScopedTypeVariables #-}

data Proxy a = Proxy
class Value a t | a -> t where value :: Proxy a -> t
instance Value True Bool where value _ = True
instance Value a t => Value (Just a) (Maybe t)
    where value _ = Just (value (Proxy :: Proxy a))

-- main = print (value (Proxy :: Proxy (Just True)))