[Haskell-beginners] Question re "Pattern match(es) are non-exhaustive"

[Frerich Raabe]

On 2015-02-10 18:01, Kim-Ee Yeoh wrote:
> On Tue, Feb 10, 2015 at 11:14 PM, Frerich Raabe <raabe at froglogic.com> wrote:
>> I noticed that a similiar case exists with
>> 
>> f :: Bool -> Bool
>> f x | x = True
>>     | not x = False

[..]

> Now you could point out that "Ah, but look at the definition of not. It 
> could inline, simplify, et voila, obtain
> 
> f True = True
> f False = False
> 
> and hence pattern-matching is complete."
> 
> Therein lies the rub. All that inlining and simplification boils down to 
> evaluating the program _in_ the compiler, so if your program diverges
> so would the compiler, which wouldn't be a happy thing.

Ah, true. The compiler would need to evaluate the program, I didn't realize 
that. But: what do you mean by 'if your program diverges' -- diverges from 
what?

-- 
Frerich Raabe - raabe at froglogic.com
www.froglogic.com - Multi-Platform GUI Testing