[Haskell-beginners] Re: Boilerplate Code

[Kyle Murphy]

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
>>>
>>>
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>>
>
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