[Haskell-cafe] The semantics of constructor patterns

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) ?