Another example

• fun is_empty nil = true | is_empty (_::_) = false;

val is_empty = fn : ‘a list -> bool

• What’s going on here?

  • What’s ‘a?

• This is a polymorphic function

  • It must take a list
  • But it can take a list of anythings
  • Those “anythings” are represented by the type variable ‘a
  • We’ll see examples where a type variable appears twice in the function type, and they must be the same

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