Haskell_ 002 Type variables

Type of the head function

 head takes a list of any type and returns the first element. The type declaration of head states that it takes a list of any type and returns one element of that type.

1. ghci> :t head  
2. head :: [a] -> a  

What is this a? Types are written in capital case, so it can't exactly be a type. It's actually a type variable. That means that a can be of any type.

polymorphic functions : Functions that have type variables.

Type of the fst function

1. ghci> :t fst  
2. fst :: (a, b) -> a  

fst takes a tuple which contains two types and returns an element which is of the same type as the pair's first component.

Because a and b are different type variables, they don't have to be different types.  It just states that the first component's type and the return value's type are the same.

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