Type variables

. Variables can be used in type expressions to represent unknown types

. Important use: check consistent use of an identifier in a language that does not require identifiers to be declared

. An inconsistent use is reported as an error

. If the variable is always used as of the same type then the use is consistent and has lead to type inference

. Type inference: determine the type of a variable/language construct from the way it is used

- Infer type of a function from its body

Type inference is the problem of determining the type of a statement from its body:

. Similar to type checking and coercion.

. But inference can be much more expressive when type variables can be used.