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)