On Wed, 12 Feb 2003, Daan Leijen wrote:
[Dean wrote:]
Warning about the use of (:=): This symbol has been proposed by John Hughes for use in some future version of Haskell to distinguish monomorphic and polymorphic `let` constructs. (See, for example, http://www.math.chalmers.se/~rjmh/Globals.ps , section 6.) You might prefer to choose a different symbol, such as (:==) or (::=).
Thanks for mentioning this. However, (:=) is a rather attractive constructor to use
Yes, indeed.
and I wonder if a monomorphic binding is used enoug to justify taking another operator away.
The choice between polymorphism and sharing is fundamental enough that I think it's worth an operator symbol (*). Of course it doesn't need to be (:=), but it shouldn't be too weird, either. [(*) The use in a binding construct is pure syntax, not really an operator. In fact, I think the same symbol could (though probably not should) be used for binding and as a general-use operator.]
It might be better after all to have a family of "let" bindings in a future haskell: let -- lazy binding let! -- strict binding let$ -- speculative binding (ie. try operationally strictly but maintain lazy semantics) let# -- monomorphic binding ?...
That proposal doesn't work, for several related reasons: 1. A `let` may introduce multiple bindings. It's undesirable that all such bindings be required to be of the same sort. 2. The same binding choices apply also to `where`. 3. The same binding choices apply also at the top level, where there's no `let` or `where` keyword to decorate. In summary, it's much cleaner to indicate the binding choice with the symbol. -- Dean