[Haskell-cafe] Partial pattern matching

[Eugene Kirpichov]

I mean, there is no way to write a firstCoord function so that it
would work, for example, on '\y -> P 42 y' and yield 42.

Except for this one:

firstCoord proj = case (proj undefined) of P x y -> x

However, this requires proj to be non-strict in its remaining argument.
But this will actually work if you pass "P x" to it for some x,
because it *is* non-strict in the remaining argument.

2009/3/9 Eugene Kirpichov <ekirpichov at gmail.com>:
> P x is indistinguishable neither in compile-time nor in run-time from
> the value \y -> P x y.
>
> And pattern matching and equality on functions is, of course, undecidable.
>
> 2009/3/9 Peter Verswyvelen <bugfact at gmail.com>:
>> In Haskell, a data constructor can be used partially applied:
>> data Pair a b = P a b
>> f = P 1
>> however, I cannot do "partial pattern matching", e.g
>> firstCoord (P x) = x
>> does not work.
>> I guess a very important reason must exist why this is the case?
>>
>>
>>
>>
>>
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>
>
>
> --
> Eugene Kirpichov
> Web IR developer, market.yandex.ru
>



-- 
Eugene Kirpichov
Web IR developer, market.yandex.ru