In section 4.3.3., chapter 4: Structured types and the semantics of
pattern-matching, by S.Peyton Jones and Philip Wadler, there is this
equation:
Eval[[\(s p1 p2 ... pt).E]] (s a1 a2 ...at) = Eval[[\p1 ... \pt.E]] a1 ...
at
The text say:
"To apply \(s p1 ... pt).E to an argument A we first evaluate A to find
out what sort of object it is."
"Finally, if A was built with the same constructor as the patern, then the
first rule applies."
What I don't get it :
(s a1 a2 ... at) must be the value of A in the semantic domain. Let call
that value a.
Then how can one know if a was built with (s a1 a2 ... at) and not with
(egg b1 b2) ?
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