[Haskell-cafe] Newbie question: mutually exclusive strict / lazy

[Daniel Fischer]

Am Samstag, 9. Februar 2008 17:33 schrieb Peter Verswyvelen:
> Consider the function
>
> cond x y z = if x then y else z
>
> I guess we can certainly say cond is strict in x.
>
> But what about y and z?
>
> If x is true,  then cond is strict in y
> If x is false, then cond is strict in z
>
> So we can't really say cond is lazy nor strict in its second or third
> argument.
>
> Of course, this is the case for many more functions, but in  the case of
> the if-then-else primitive, does the strictness analyzer make use of this
> "mutually exclusive strictness" fact?
>
> Cheers,
> Peter

Hope I remember correctly...

A function is strict in an argument, if whenever that argument is _|_, the 
result is _|_, regardless of possible other arguments.

Since 
if True then 0 else _|_ == 0,
if-then-else is nonstrict in the third argument, similarly
if False then _|_ else 0 == 0,
so if-then-else is nonstrict in the second argument.

Cheers,
Daniel