[Haskell-cafe] The semantics of constructor patterns
cristian.baboi at gmail.com
Sun Dec 30 12:18:29 EST 2007
On Sun, 30 Dec 2007 19:13:47 +0200, Jonathan Cast
<jonathanccast at fastmail.fm> wrote:
> 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
>> 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.
I have not seen that definition in the book.
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