[Haskell-beginners] How to avoid evaluating the second (undefined) argument of a Boolean AND operation?

[Jack Henahan]

You're right, of course. I totally spaced out while reading the error message. That'll teach me to reply to lists while on the phone. :/

On Jun 22, 2011, at 7:21 PM, Felipe Almeida Lessa wrote:

> On Wed, Jun 22, 2011 at 8:07 PM, Jack Henahan <jhenahan at uvm.edu> wrote:
>> Use
>> 
>>    myAND F _ = F
>> 
>> instead of using x.
> 
> Actually that doesn't change anything and both definitions are the
> same thing.  The problem here is that "type checking is not lazy" =).
> The second argument of myAND in "myAND F (1/0)" isn't being evaluated
> at all; actually your code isn't running at all.  What is being
> reported is a type error, not a runtime error.  Try one of these
> instead:
> 
>  myAND F undefined
>  myAND F (error "foo")
>  myAND F (let x = x in x)
>  myAND F (unsafePerformIO $ launchMissiles)
> 
> Cheers! =)
> 
> -- 
> Felipe.

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