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