Quoting Gabriel Dos Reis <[EMAIL PROTECTED]>:
On Wed, 9 May 2007, Bill Page wrote:
[...]
| > > ... in Haskell, by a kind of convenient abuse of notation
| > > (or polymorphism if you wish) 'MkInt' also denotes a
| > > function
| > >
| > > MkInt: Int -> MkInt Int
| > >
| > > that creates an object of type 'MkInt Int' from an object
| > > in 'Int'. I think this is a potential source of confusion.
|
| Probably I should have avoided the wording above that might
| (incorrectly) be interpreted as a slur against Haskell.
Well, I did not interepret it as a slur against Haskell. However, it did
appear to me to be fundamentally incorrect to miss the core ideas of
algebraic data typea and how they lead to GADT:
data Expr where
MkInt:: Integer -> Expr
MkAdd:: Expr -> Expr -> Expr
MkMul:: Expr -> Expr -> Expr
That is all that is needed to define more operations on values of
type Expr.
I agree that GADT is important in the context of Haskell which has
(in principle) a complete and formal semantics. But I do not think it
is so interesting in Spad and Aldor which already has the full
machinery of abstract data types. If we want to add more operations
on a given type, we have 'add inheritance', e.g.
NewExpr: ExprCat with ... = Expr add ...
and even post-facto extension of domains in Aldor.
I can see however why this is interesting from the point of view of
Boot. If Boot could be given a formal semantics like Haskell, then
one might be much more confident of it's use to implement Spad
and therefore ultimately confidence in the semantics of Spad.
Am I wrong in presuming this underlying motivation? But if you still
think I am just confused, please don't feel obliged to say more than
that you think so...
Regards,
Bill Page.
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