Question

What is the recommended way of testing several boolean expressions ?

I’ve been using this:

case () of () | test1 -> value1
              | test2 -> value2
              | otherwise -> value3

Is this good style ? is there a prettier way ?

Answer 1

This pattern can be simulated with a function — for instance, cond from Control.Conditional:

signum x = cond [(x > 0     ,  1)
                ,(x < 0     , -1)
                ,(otherwise ,  0)]

I can’t call it particularly beautiful, though.

---

In next GHC we will be able to use multi-way if, hooray! (just found it out)

f t x = if | l <- length t, l > 2, l < 5 -> "length is 3 or 4" 
           | Just y <- lookup x t        -> y 
           | False                       -> "impossible" 
           | null t                      -> "empty"