[Haskell-beginners] How to avoid evaluating the second (undefined) argument of a Boolean AND operation?
Jack Henahan
jhenahan at uvm.edu
Thu Jun 23 01:07:45 CEST 2011
Use
myAND F _ = F
instead of using x.
On Jun 22, 2011, at 7:03 PM, Costello, Roger L. wrote:
> Hi Folks,
>
> Here is my own version of the Bool datatype, and my own version of the Boolean AND:
>
> data MyBool = F | T
>
> 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:
>
> No instance for (Fractional MyBool)
> arising from a use of `/'
> Possible fix: add an instance declaration for (Fractional MyBool)
> In the second argument of `myAND', namely `(1 / 0)'
> In the expression: myAND F (1 / 0)
> In the definition of `t4': t4 = myAND F (1 / 0)
>
> Why does it evaluate the second argument when the answer is already known from the first argument?
>
> How can I design it so that if the answer is known from the first argument, then an undefined second argument doesn't produce an error?
>
> /Roger
>
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