[Haskell-beginners] Re: Boilerplate Code

[Kyle Murphy]

You're partially right. The Typeable is redundant because Data has the type:

(Typeable a) => Data a

-R. Kyle Murphy
--
Curiosity was framed, Ignorance killed the cat.


On Tue, Aug 3, 2010 at 15:01, Ozgur Akgun <ozgurakgun at gmail.com> wrote:

> 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
>>>>> Beginners at haskell.org
>>>>> http://www.haskell.org/mailman/listinfo/beginners
>>>>>
>>>>>
>>>>
>>>
>>
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>>
>
>
> --
> Ozgur Akgun
>
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