資料來源 : Free On-Line Dictionary of Computing

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)