[Haskell-beginners] no instance for monad

[Emmanuel Touzery]

and the subject of the mail is because if actually give parameters to the
print call I get a different error message, which is the first one I saw:

 No instance for (MonadIO ((->) t0))
      arising from a use of `TF.hprint'
    Possible fix: add an instance declaration for (MonadIO ((->) t0))

but here I might be misusing the library in yet another way. I would first
focus on the "no parameters" case.

Emmanuel

On Sat, Oct 20, 2012 at 8:24 PM, Emmanuel Touzery <etouzery at gmail.com>wrote:

> Hello,
>
>  in my program i have a function which can be simplified like this:
>
>  writeProgramInfo :: Handle -> ProgramInfo -> IO ()
>  writeProgramInfo handle program = do
>         hPrintf handle "hello"
>
> This is using the hPrintf from Text.Printf.
>
> Unfortunately that hPrintf works on String parameters which is not good
> enough because I need unicode character support (if I use unpack it works
> on linux but complains about invalid characters on windows, I guess because
> my locale on linux is utf-8 but on windows it's a local codepage). So I
> found the Text.Format library:
>
> http://hackage.haskell.org/packages/archive/text-format/0.3.0.7/doc/html/Data-Text-Format.html
>
>  Which appears to solve exactly that problem (out of the box Text
> support). However I can't just install the library, add the import
> statements, and replace hPrintf by TF.hprint and change the format string,
> that doesn't work; if I change this function to:
>
> import qualified Data.Text.Format as TF
>
> writeProgramInfo :: Handle -> ProgramInfo -> IO ()
> writeProgramInfo handle program = do
>         TF.hprint handle "hello"
>
> I get this error message at build time:
>
> Couldn't match expected type `IO ()'
>                 with actual type `ps0 -> m0 ()'
>     In the return type of a call of `TF.hprint'
>     In the expression: TF.hprint handle "hello"
>     In the expression: do { TF.hprint handle "hello" }
>
> Now this clearly results from the type of hprint, which is:
> hprint :: (MonadIO<http://hackage.haskell.org/packages/archive/transformers/0.2.2.0/doc/html/Control-Monad-IO-Class.html#t:MonadIO>m,
> Params<http://hackage.haskell.org/packages/archive/text-format/0.3.0.7/doc/html/Data-Text-Format-Params.html#t:Params>ps) =>
> Handle<http://hackage.haskell.org/packages/archive/base/4.4.1.0/doc/html/GHC-IO-Handle.html#t:Handle>->
> Format<http://hackage.haskell.org/packages/archive/text-format/0.3.0.7/doc/html/Data-Text-Format.html#t:Format>-> ps -> m
> ()<http://hackage.haskell.org/packages/archive/ghc-prim/0.2.0.0/doc/html/GHC-Unit.html#t:-40--41->
>
>  while the type of hPrintf is:
> hPrintf<http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/Text-Printf.html#v%3AhPrintf>::
> HPrintfType<http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/Text-Printf.html#t%3AHPrintfType>r =>
> Handle<http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/GHC-IO-Handle.html#t%3AHandle>->
> String<http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/Data-Char.html#t%3AString>-> r
>
>  with the precision: The return type is restricted to (IO<http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/System-IO.html#t%3AIO>a)
> .
>
>  Well I didn't completely 'get' the monad thing yet. I think I'm pretty
> close.. but not there yet. I'm pretty sure understanding this specific
> example will take me one step closer... What am I missing here?
>
>  Thank you!
>
> Emmanuel
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/beginners/attachments/20121020/f2bbaedc/attachment.htm>