[Haskell-cafe] Re: Scary type inference for monadic function definitions

Ahn, Ki Yung kyagrd at gmail.com
Thu Jun 4 01:56:56 EDT 2009

Ahn, Ki Yung 쓴 글:
> Scary type inference for monadic function definitions
> (or, why you'd want to annotate types for monadic function definitions)
> This is a real example that I've experienced.
> I defined the following function.
>> checkOneVerseByLineWith readLine v =
>>   do mg <- readLine
>>      case mg of
>>        Just g  -> return Just (v==g)
>>        Nothing -> return Nothing
> My intention was to use it something like this:
> checkOneVerseByLineWith (readline "% ")
> where readline is the library function from System.Console.Readline.
> As you can see, there is an obvious mistake which I forgot to
> group (Just (v==g)) in parenthesis.  However, GHC or any other
> Haskell 98 compliant implementation will infer a type for you
> and this will type check!  Try it yourself if in doubt.
> Of course, checkOneVerseByLineWith (readline "% ") won't type check
> because checkOneVerseByLineWith has strange type.  The reason why
> the above definition type checks is because ((->) r) is an instance
> of Monad.

Oh, I happened to be importing Control.Monad.Trans somehow,
it just doesn't work with Prelude import itself.
And, there were already some discussions on this last year:

[Haskell-cafe] The danger of Monad ((->) r)
Tomasz Zielonka
Tue, 15 May 2007 03:05:30 -0700

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