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Computing (FOLDOC) dictionary
principal type
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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.