[Haskell-beginners] What is the Haskell idiom for "if then else if then"

[Daniel Fischer]

Am Montag 30 März 2009 17:33:02 schrieb Peter Hickman:
> I've just written this piece of code
>
>      if r < 0
>      then LEFT
>      else
>          if r > 0
>          then RIGHT
>          else STRAIGHT
>
> which works just fine but it does look rather clunky (as it would in
> any language I suppose). Is there a Haskell idiom for this type of code?

Usually, people use guards:

function r
    | r < 0     = LEFT
    | r > 0     = RIGHT
    | otherwise = STRAIGHT

Another option would be a case:

    case compare r 0 of
      LT -> LEFT
      GT -> RIGHT
      EQ -> STRAIGHT

You can also have a case with guards:

case expression of
    pattern1 | condition1.1 -> result1.1
             | condition1.2 -> result1.2
    pattern2 | condition2.1 -> result2.1
    pattern3 -> result3
    _ -> defaultresult