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I am attempting to design a class to use as a basis for general purpose graphs.  my basic design looks as follows:

type 'a graph_edge = class

val from_node : 'a Option

val to_node : 'a Option

new () = { from_node = None; to_node = None; }

new (from_n : 'a, to_n:'a) = { from_node = Some from_n; to_node = Some to_n; }

type ('a, #graph_edge<'a>) graph_base = class

val nodes : 'a array

val edges : #graph_edge<'a> array

I have found a construct that seems to work.  At least I am getting no syntax errors when using it. 

type ('a, 'b)graph_base when 'b :> graph_edge<'a> = class

val nodes : 'a array

val edges : 'b array

I have a question the dreaded syntax error is extremely uninformative.  is it possible to enhance the compiler error reporting so that when a syntax error occurs the actual syntactical law that is being violated could be cited?

Thanks David

Hi David,

We can improve the error messages on a case by case basis by adding rules to the grammar. Please drop us a bug report to fsbugs AT microsoft DOT com each time you see and error message that looks like it can be improved.

thanks!

don