[Haskell-cafe] The semantics of constructor patterns

Benja Fallenstein benja.fallenstein at gmail.com
Sun Dec 30 12:23:00 EST 2007

Hi Cristian,

On Dec 30, 2007 6:10 PM, Cristian Baboi <cristian.baboi at gmail.com> wrote:
> 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) ?

Because the semantic domain is chosen so that (s a1 a2 ... at) and
(egg b1 b2) are distinct objects.

More precisely, the domain corresponding to (for example) the type

data T = C1 T11 T12 | C2 T21 T22

should be isomorphic to the domain

[[T]] = lift (([[T11]] * [[T12]]) + ([[T21]] * [[T22]]))

where * is cartesian product, + is disjoint sum (e.g., X+Y = {(1,x) |
x in X} union {(2,y) | y in Y}, and 'lift' adds the bottom element to
the domain.

So here, C1 and C2 correspond to the two sides of the disjoint sum,
meaning you can tell them apart.

Hope that helps?

- Benja

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