[Haskell-cafe] type error

[Ryan Ingram]

The other simple option is to eta-expand:
> let myprint x = print x
will get the correct type,

The monomorphism restriction is to stop things that look like *values*
whose computation results are memoized, from turning into *functions*
which need a dictionary context and get recomputed every time they are
accessed.  In your declaration, "myprint" looks like a value, so it
should only get computed once.  In order to do this it needs to be
monomorphic, because otherwise it could have a different value
depending on where it was used.

 -- ryan

On Wed, Dec 10, 2008 at 10:55 PM, Janis Voigtlaender
<voigt at tcs.inf.tu-dresden.de> wrote:
> Cetin Sert wrote:
>>
>> Hi,
>>
>> Why does this not function?
>>
>> Prelude> sequence [print 'a', print 2]
>> 'a'
>> 2
>> [(),()]
>> *Prelude> let myprint = print*
>> *Prelude> sequence [myprint 'a', myprint 2]*
>>
>> <interactive>:1:18:
>>    Couldn't match expected type `()' against inferred type `Char'
>>    In the first argument of `myprint', namely 'a'
>>    In the expression: myprint 'a'
>>    In the first argument of `sequence', namely
>>        `[myprint 'a', myprint 2]'
>
> The problem is the monomorphism restriction:
>
> ~> ghci
> GHCi, version 6.8.2: http://www.haskell.org/ghc/  :? for help
> Loading package base ... linking ... done.
> Prelude> :t print
> print :: (Show a) => a -> IO ()
> Prelude> let myprint=print
> Prelude> :t myprint
> myprint :: () -> IO ()
> Prelude> :q
> Leaving GHCi.
> ~> ghci -fno-monomorphism-restriction
> GHCi, version 6.8.2: http://www.haskell.org/ghc/  :? for help
> Loading package base ... linking ... done.
> Prelude> let myprint=print
> Prelude> :t myprint
> myprint :: (Show a) => a -> IO ()
>
>> Can providing some type annotations (interactively in ghci or in some .hs
>> file) help solve the problem?
>
> Yes.
>
> --
> Dr. Janis Voigtlaender
> http://wwwtcs.inf.tu-dresden.de/~voigt/
> mailto:voigt at tcs.inf.tu-dresden.de
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