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From: | Ralf Hemmecke |
Subject: | Re: [Axiom-mail] Spad and inductive types |
Date: | Wed, 09 May 2007 13:36:02 +0200 |
User-agent: | Thunderbird 2.0.0.0 (X11/20070326) |
Dear Bill, I only have a comment on the following.
I agree that it's a source of confusion. But I must admit that I would have written MkInt: Int -> Expr instead. Otherwise there would be a type mismatch in eval (MkInt i) = i since MkInt(Int) is not equal to Expr. Don't you agree?'MkInt i' is a member of the type 'MkInt Integer' which is a subtype of Expr. The expression eval (MkInt i) = i only defines eval over this subtype of Expr. The complete definition of 'eval' also requires two other definitions for 'MkAdd Expr Expr' and 'MkMul Expr Expr', respectively.
The "subtype" is the thing that bothers me. Not in Haskell, but in your Aldor code on
http://wiki.axiom-developer.org/SandBoxAldorInductiveTypes You define a domain MkInt(Z:IntegerNumberSystem): ExprCat == add ... and then you use it in Expr: ExprCat == add { Rep == Union(Mkint:MkInt(Integer), ...); ... }I don't think that construction qualifies MkInt to count as a subtype of Expr. Note that the representations of MkInt and Expr are different.
http://aldor.org/docs/HTML/chap7.html#5 (Although I find that section not very illuminating w.r.t. domains.) Ralf
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