Note that, unfortunately, GHC's exhaustiveness checker is *not* good enough to figure out that your predicates are covering. :o) Perhaps there is an improvement to be had here. Edward Excerpts from Albert Y. C. Lai's message of Mon Apr 22 00:51:46 -0700 2013:

When I was writing http://www.vex.net/~trebla/haskell/crossroad.xhtml I wanted to write: branching on predicates and then using selectors is less efficient than pattern matching, since selectors repeat the tests already done by predicates.

It is only ethical to verify this claim before writing it. So here it goes, eval uses pattern matching, fval uses predicates and selectors:

module E where

data E = Val{fromVal::Integer} | Neg{fromNeg::E} | Add{fromAdd0, fromAdd1 :: E} isVal Val{} = True isVal _ = False isNeg Neg{} = True isNeg _ = False isAdd Add{} = True isAdd _ = False

eval (Val n) = n eval (Neg e0) = - eval e0 eval (Add e0 e1) = eval e0 + eval e1

fval e | isVal e = fromVal e | isNeg e = - fval (fromNeg e) | isAdd e = fval (fromAdd0 e) + fval (fromAdd1 e)

Simple and clear. What could possibly go wrong!

$ ghc -O -c -ddump-simpl -dsuppress-all -dsuppress-uniques E.hs

...

Rec { fval fval = \ e -> case e of _ { Val ds -> ds; Neg ds -> negateInteger (fval ds); Add ipv ipv1 -> plusInteger (fval ipv) (fval ipv1) } end Rec }

Rec { eval eval = \ ds -> case ds of _ { Val n -> n; Neg e0 -> negateInteger (eval e0); Add e0 e1 -> plusInteger (eval e0) (eval e1) } end Rec }

Which of the following best describes my feeling? [ ] wait, what? [ ] lol [ ] speechless [ ] oh man [ ] I am so pwn3d [ ] I can't believe it [ ] what can GHC not do?! [ ] but what am I going to say in my article?! [ ] why is GHC making my life hard?! [X] all of the above