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Today's Topics:
1. Re: When a return type constrained by class (Haisheng Wu)
----------------------------------------------------------------------
Message: 1
Date: Sun, 18 Mar 2012 18:24:20 +0800
From: Haisheng Wu <fre...@gmail.com>
Subject: Re: [Haskell-beginners] When a return type constrained by
class
To: Kyle Murphy <orc...@gmail.com>, allber...@gmail.com
Cc: Haskell Beginer <beginners@haskell.org>
Message-ID:
<cafj8lzek-afk1rrxjo-4qst7ragk4nmi0l+vur84afquace...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
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 <orc...@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 <fre...@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
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners@haskell.org
>> http://www.haskell.org/mailman/listinfo/beginners
>>
>>
>
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