Syntax in Functions

Pattern matching

When defining functions, you can define separate function bodies for different patterns. This leads to really neat code that's simple and readable.

lucky :: (Integral a) => a -> String
lucky 7 = "LUCKY NUMBER SEVEN!"
lucky x = "Sorry, you're out of luck, pal!"
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Here is a more useful example with factorials:

factorial :: (Integral a) => a -> a
factorial 0 = 1
factorial n = n * factorial (n - 1)
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Note: Pattern matching can also fail if the fallback case is not defined.

Pattern matching can also happen with list comprehensions:

ghci> let xs = [(1,3), (4,3), (2,4), (5,3), (5,6), (3,1)]
ghci> [a+b | (a,b) <- xs]
[4,7,6,8,11,4]
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Guards

bmiTell :: (RealFloat a) => a -> String
bmiTell bmi
    | bmi <= 18.5 = "You're underweight, you emo, you!"
    | bmi <= 25.0 = "You're supposedly normal. Pffft, I bet you're ugly!"
    | bmi <= 30.0 = "You're fat! Lose some weight, fatty!"
    | otherwise   = "You're a whale, congratulations!"
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An example of a small myCompare function:

myCompare :: (Ord a) => a -> a -> Ordering
a `myCompare` b
    | a > b     = GT
    | a == b    = EQ
    | otherwise = LT
-- 3 `myCompare` 2 -> GT
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Where bindings

bmiTell :: (RealFloat a) => a -> a -> String
bmiTell weight height
    | bmi <= 18.5 = "You're underweight, you emo, you!"
    | bmi <= 25.0 = "You're supposedly normal. Pffft, I bet you're ugly!"
    | bmi <= 30.0 = "You're fat! Lose some weight, fatty!"
    | otherwise   = "You're a whale, congratulations!"
    where bmi = weight / height ^ 2
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Let bindings