PR Stanley <[EMAIL PROTECTED]> wrote:
> Hi
> isZero :: Int -> Bool
> isZero 0 = True
> isZero n | n /= 0 = False
>
> The order in which the above equations appear makes no difference to
> the application of isZero. Does the Haskell interpreter rewrite
> patterns into one single definition using some sort of switch or if
> construct? Why does an equation without a guard have to be placed
> after the more specific cases?To put it another way, why doesn't the
> interpreter identify the more specific cases and put them before the
> general ones.
> Cheers
> Paul
>
For completeness' sake:
isZero 0 = True
isZero _ = False
works, and, contrary to your example, requires an order to
have a well defined meaning.
It's equivalent[1] to
isZero n = case n of
0 -> True
_ -> False
, and yours to
isZero n = case n of
0 -> True
n -> if n /= 0
then False
else undefined
I'll leave the last question unanswered, just try to write such a beast.
PS: I prefer
isZero = (==) 0
[1] which doesn't mean that one is reduced to the other. It just means
they're semantically identical.
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