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

[Brandon Allbery]

On Wed, Jun 22, 2011 at 19:03, Costello, Roger L. <costello at mitre.org>wrote:

> myAND :: MyBool -> MyBool -> MyBool
> myAND F x = F
> myAND T x = x
>
> If the first argument is F then return F. I assumed that the second
> argument would not even bother being evaluated.
>
> I figured that I could provide an undefined value for the second argument:
>
>     myAND F (1 / 0)
>
> However, that doesn't work. I get this error message:
>

There's no evaluation here.  Haskell does no implicit coercions for you
(with a limited exception involving the definition of numeric literals) so
you are passing a Fractional a => a (1/0) when a MyBool is expected, and
MyBool isn't a Fractional.  (No coercions means, in this case, that a number
*cannot* be used as a boolean, neither a real one nor your alternative
formulation, without some kind of explicit coercion.)

How you fix this depends on what you're trying to accomplish.  If you really
want to use a division by zero there, you need to use an Integral a => a and
coerce it manually:

> myAND F (toEnum (1 `div` 0))

or, if you insist on Fractional,

> myAND F (toEnum (fromRational (1 / 0)))

...and both of these require that you declare MyBool as

> data MyBool = F | T deriving Enum

so that you can use toEnum.

If you're just trying to prove the point about lazy evaluation, leave the
numbers out of it:

> myAND F undefined

("undefined"'s type is "a", that is, any type; from this you can infer that
it can never produce an actual value, it can only raise an exception,
because there is no value that inhabits all possible types.)

Alternately you can say

> myAND F (error "wait, what?")

which lets you control what message is printed if for some reason the second
parameter is evaluated.

-- 
brandon s allbery                                      allbery.b at gmail.com
wandering unix systems administrator (available)     (412) 475-9364 vm/sms
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/beginners/attachments/20110622/78902e3c/attachment.htm>