[Haskell-beginners] How to avoid evaluating the second (undefined) argument of a Boolean AND operation?
Costello, Roger L.
costello at mitre.org
Thu Jun 23 01:03:17 CEST 2011
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?
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