A type variable is a way to refer to an unspecified type or set of types in Haskell type signatures. Type variables ordinarily will be equal to themselves throughout a type signature. Let us consider some examples:
id :: a -> a
The only type variable, a, occurs twice, once as an argument, once as a result. The type variable is parametrically polymorphic and could be strictly anything.
(+) :: Num a => a -> a -> a
Only one type variable named a, again. a is constrained to requiring an instance of Num. Two arguments of the same type a and one result.
Type variables only appear in type signatures. (that may seem obvious)
A type variable is a variable used in a type signatures such as a in f :: a -> a.
A type variable is a lower case letter such as a used in a type signature.
The only type variable,
a, occurs twice, once as an argument, once as a result. The type variable is parametrically polymorphic and could be strictly anything.Only one type variable named
a, again.ais constrained to requiring an instance of Num. Two arguments of the same typeaand one result.ainf :: a -> a.aused in a type signature.