S.D.Mechveliani, you wrote:
>
> will you, please, analyze the following suggestion.
> Let we call it the
> extended type context attachement.
This feature -- specialization, or partial specialization, of polymorphic
functions -- is quite important. Alexander Stepanov (designer of the C++
Standard Template Library) has been explaining this on a recent thread
crossposted to comp.lang.functional amoung others.
I haven't thought about it too hard, but I have thought about it a bit,
and I don't see any way of achieving it without either (a) basically giving
up separate compilation or (b) requiring expensive computations at runtime
to figure out which specialization to invoke. (C++ chose (a).)
> Suggestion:
>
> ------------------------------------------------------------------
> to make it possible to srcipt like this:
>
> det :: AddMul a => Matrix a -> a
> det mM = expandByRow mM where ...
>
> det :: (AddMul a,Div a) => Matrix a -> a
> det mM = gaussElimination mM
> where ...
I don't think this syntax is going to work. For example, how does it
interact with separate compilation? I think the syntax should explicitly
indicate that the second definition is a specialization.
Here's another possible alternative way of achieving a similar effect:
det :: AddMul a => Matrix a -> a
det mM | class mM Div = expandByRow mM where ...
det mM | otherwise = gaussElimination mM where ...
Here `class <expr> <type-class>' is a new builtin boolean expression
that returns true iff the expression has a type that is an instance
of the specified type-class. [Or perhaps it should be written as
`instance <type-class> (type_of <expr>)', e.g. `instance Div (type_of mM)'?.
That would scale better to multi-parameter type classes. In case you
hadn't noticed yet, I'm making this up as I go along.]
This alternative is still going to be difficult to implement
in the presence of separate compilation.
--
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WWW: <http://www.cs.mu.oz.au/~fjh> | of excellence is a lethal habit"
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