Question

I’ve created a function in which I can use (as far as I know) case expressions or guards.

foo a b c = case a of 1 -> [...]
                      2 -> [...]
                      3 -> [...] 
                      [...]
                      otherwise -> error "..."

or

foo a b c | a == 1 = [...]
          | a == 2 = [...]
          | a == 3 = [...] 
          | [...]
          | otherwise = error "..."

So, the question is: which of those 2 (case or guards) are “better” coding? Are both basically the same?

Answer 1

The first one is considered better style, for 2 reasons.

First of all: Many people would say that it looks better, since you don’t have to type out all of the ==. This is a very subjective reason, of course. Also, you would normally not even introduce a new case statement, but just match the arguments in the function argument list like so:

foo 1 b c = ... -- etc
...
foo _ b c = ... -- for the "otherwise" part

This makes the code even more compact and readable, which many people like.

Secondly, there actually is a semantic difference. Imagine that you have a data type like this:

data Cake = Apple | Cheese | Cream

If you use the first method, you match against the constructors in the case..of expression:

case a of
  Apple -> "fruit"
  _     -> "not fruit"

However, if you try to do a guarded expression of some sort, like this:

| a == Apple = "fruit"
| otherwise  = "not fruit"

… it won’t actually work, because the Cake type doesn’t have an Eq instance, so you can’t use == to compare two values. Introducing an Eq instance (with deriving (Eq) after the data definition) is not always wanted, so not having to do it in this case might be significant.