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@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@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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