principal type

The most general type of an expression. For example, the

following are all valid types for the lambda abstraction

( x . x):

Int -@# Int

Bool -@# Bool

(a-@#b) -@# (a-@#b)

but any valid type will be an instance of the principal type:

a -@# a. An instance is derived by substituting the same type

expression for all occurences of some type variable. The

principal type of an expression can be computed from those of

its subexpressions by Robinson's unification algorithm.

(1994-12-16)