Rob,
Considering then ML semantics is so carefully defined the syntax is
really a mess. There are lots of informal comments which make it really
quite difficult to understand exactly what is allowed and what is not.
The ML90 definition says "For each value binding pat = exp within rec,
exp must be of the form fn match, possibly constrained by one or more
type expressions". The ML97 definition removed the final clause so
implying that type constraints were no longer allowed.
Poly/ML has always allowed type constraints on recursive bindings and
also, by default, parentheses. A recent change has meant that
parentheses are now included in the parse tree which is why the check
failed. Because there doesn't seem to be any logical reason to exclude
parentheses I've now changed the check to allow parentheses.
By the way, a type constraint is allowed on a fun binding but it applies
only to the result type not to the type of the function as a whole.
Regards,
David
Rob Arthan wrote:
This looked like a really useful feature so I thought I'd try it out.
Unfortunately, I have code that fails as in the cut-down example below:
Poly/ML 5.3 Enhanced Reporting Testing
> val rec f = (fn x => ());
Error-Recursive declaration is not of the form `fn match'
Found near val rec f = (fn x => ())
Static Errors
If this is intended, I presume the enhanced reporting includes a check
that disallows the brackets around the right-hand side of the
declaration. I note that hamlet also says that this is an error, so
there is support for reading the standard that way. I suspect I will not
be alone in having a fair amount of code that doesn't conform.
This looks like another place where Standard ML imposes unintuitive
constraints that make it hard to write readable, maintainable code. The
reason for using "val rec" rather than "fun" in the real code is that I
want to give a type constraint on the function (which for no obvious
reason is not allowed in a "fun" binding); the reason for the brackets
is that the declaration is long and I want to be able to use bracket
matching to find the end.
_______________________________________________
polyml mailing list
polyml@inf.ed.ac.uk
http://lists.inf.ed.ac.uk/mailman/listinfo/polyml
