# [Haskell-cafe] The semantics of constructor patterns

Jonathan Cast jonathanccast at fastmail.fm
Sun Dec 30 12:13:47 EST 2007

On 30 Dec 2007, at 11:10 AM, Cristian Baboi wrote:

> 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) ?

I suspect that s and egg are constructors, in which case their images
are (by definition) mutually exclusive.

jcc