[Haskell-beginners] When a return type constrained by class

[Haisheng Wu]

Thank you Kyle and Brandon!

I am actually re-implementing a toy solution which was done in JavaScript.
Turns out I can not make quite clear difference between typeclasses and OOP
thus I got this question.

I'm also aware making a class for nextGuess would be helpful and I'm still
working on.

PS: you might like to find the toy program at :
https://github.com/freizl/play-hangman-game

-Haisheng


On Sun, Mar 18, 2012 at 5:13 PM, Kyle Murphy <orclev at gmail.com> wrote:

> I'd guess the problem is because nextGuess isn't capable of producing any
> instance of Guess. The type signature of a function isn't just to provide
> callers information about a function like in most languages, it's also a
> contract the function itself must abide by. In contrast it's common in an
> OO language like Java to provide a more general return type like an
> interface or superclass in the method declaration, and then to return a
> more concrete type in the actual method. Haskell in contrast requires that
> you be capable of returning *anything* that your functions type signature
> claims it can. In your example, nextGuess must be capable of producing
> *any* instance of nextGuess that a caller of the function requests. Put
> another way, both of these must be valid:
>
> nextGuess "example" :: GuessLetter
> nextGuess "example" :: GuessWord
>
> Typically this is accomplished by making a function like nextGuess part of
> the class as in how Read does it:
>
> class Read a where
>   read :: Read a => String -> a
>
> By definition then, any instance of read must provide a read function
> capable of producing something of that type. So, it seems obvious the
> solution to your problem is to make nextGuess part of the Guess class.
> Alternatively if that's not really what you're trying to accomplish the
> type signature of nextGuess is probably wrong. If you provide more detail
> on what you're attempting to do maybe someone on here can suggest another
> way of accomplishing it.
>
> -R. Kyle Murphy
> --
> Curiosity was framed, Ignorance killed the cat.
>
>
> On Sun, Mar 18, 2012 at 02:23, Haisheng Wu <freizl at gmail.com> wrote:
>
>>  Hi there,
>>  Do you have any idea why the following code is not valid?
>>
>> class Guess g where
>>  makeGuess :: g -> String
>>
>> instance Guess GuessLetter where
>>  makeGuess = ...
>>
>> instance Guess GuessWord where
>>  makeGuess = ...
>>
>> *-- | This function is nod valid *
>> nextGuess :: Guess g => String -> g
>> nextGuess = ...
>>
>>
>> Thank you.
>>
>> -Haisheng
>>
>>
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>>
>
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