[Haskell-beginners] Re: Boilerplate Code

[Ozgur Akgun]

Why do you need them to be Typeable? toConstr has the following type:

toConstr :: (Data a) => a -> Constr

Best,

On 3 August 2010 19:50, Kyle Murphy <orclev at gmail.com> wrote:

> I was close, this actually does what was asked:
>
> import Data.Data
>
> typeChecker :: (Typeable a, Typeable b, Data a, Data b) => a -> b -> Bool
> typeChecker a b = toConstr a == toConstr b
>
>
> -R. Kyle Murphy
> --
> Curiosity was framed, Ignorance killed the cat.
>
>
> On Tue, Aug 3, 2010 at 14:42, Kyle Murphy <orclev at gmail.com> wrote:
>
>> Actually looking at the original question I'm not sure my code does what
>> was intended. I was looking at does some type (a b) == (a c), which wasn't
>> exactly the question. Oh well, back to the drawing board.
>>
>>
>> -R. Kyle Murphy
>> --
>> Curiosity was framed, Ignorance killed the cat.
>>
>>
>> On Tue, Aug 3, 2010 at 14:38, Kyle Murphy <orclev at gmail.com> wrote:
>>
>>> Less of a dirty dirty hack (requires that SchemeVal be an instance of
>>> Typeable):
>>>
>>> import Data.Typeable
>>> import Data.Maybe
>>>
>>> typeChecker :: (Typeable a, Typeable b) => a -> b -> Bool
>>> typeChecker a b = f a == f b
>>>         where
>>>                 f :: (Typeable a) => a -> Maybe TypeRep
>>>                 f = listToMaybe . typeRepArgs . typeOf
>>>
>>>
>>> -R. Kyle Murphy
>>> --
>>> Curiosity was framed, Ignorance killed the cat.
>>>
>>>
>>>
>>> On Tue, Aug 3, 2010 at 13:51, Alex Rozenshteyn <rpglover64 at gmail.com>wrote:
>>>
>>>> That is a dirty, dirty hack.
>>>>
>>>>
>>>> On Tue, Aug 3, 2010 at 8:45 PM, Christian Maeder <
>>>> Christian.Maeder at dfki.de> wrote:
>>>>
>>>>> Matt Andrew schrieb:
>>>>> > Hi all,
>>>>> >
>>>>> > I am in the process of writing a Scheme interpreter/compiler in
>>>>> Haskell as my first serious project after learning the basics of Haskell.
>>>>> The goal is to really get a feel for Haskell. I am trying to accomplish this
>>>>> as much as I can on my own, but am referring to Jonathan Tang's 'Write
>>>>> Yourself a Scheme in 48 hours' whenever I get really stuck.
>>>>> >
>>>>> > I have a question regarding a pattern that I have found within my
>>>>> code for which I cannot seem to find an abstraction.
>>>>> >
>>>>> > I am implementing some of the primitive Scheme type-checker functions
>>>>> with the following code:
>>>>> >
>>>>> > numberP :: SchemeVal -> SchemeVal
>>>>> > numberP (Number _) = Bool True
>>>>> > numberP _          = Bool False
>>>>> >
>>>>> > boolP :: SchemeVal -> SchemeVal
>>>>> > boolP (Bool _) = Bool True
>>>>> > boolP _        = Bool False
>>>>> >
>>>>> > symbolP :: SchemeVal -> SchemeVal
>>>>> > symbolP (Atom _) = Bool True
>>>>> > symbolP _        = Bool False
>>>>> >
>>>>> > This is a pattern that I could easily provide an abstraction for with
>>>>> a Lisp macro, but I'm having trouble discovering if/how it's possible to do
>>>>> so elegantly in Haskell. The closest (but obviously incorrect) code to what
>>>>> I'm trying to accomplish would be:
>>>>> >
>>>>> > typeChecker :: SchemeVal -> SchemeVal -> SchemeVal
>>>>> > typeChecker (cons _) (cons2 _) = Bool $ cons == cons2
>>>>> >
>>>>> > I understand this code drastically misunderstands how pattern
>>>>> matching works, but (hopefully) it expresses what I'm trying to accomplish.
>>>>> Anyone have any suggestions?
>>>>>
>>>>> typeChecker s1 s2 = let f = takeWhile isAlphaNum . show in
>>>>>   Bool $ f s1 == f s2
>>>>>
>>>>> hoping that my "f" just extracts the constructor as string.
>>>>>
>>>>> C.
>>>>>
>>>>> > I do realise that such an abstraction is barely worth it for the
>>>>> amount of code it will save, but this exercise is about learning the ins and
>>>>> outs of Haskell.
>>>>> >
>>>>> > Appreciate you taking the time to read this,
>>>>> >
>>>>> > Matt Andrew
>>>>> _______________________________________________
>>>>> Beginners mailing list
>>>>> Beginners at haskell.org
>>>>> http://www.haskell.org/mailman/listinfo/beginners
>>>>>
>>>>
>>>>
>>>>
>>>> --
>>>> ()  ascii ribbon campaign - against html e-mail
>>>> /\  www.asciiribbon.org   - against proprietary attachments
>>>>
>>>>           Alex R
>>>>
>>>>
>>>> _______________________________________________
>>>> Beginners mailing list
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>>>> http://www.haskell.org/mailman/listinfo/beginners
>>>>
>>>>
>>>
>>
>
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-- 
Ozgur Akgun
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