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Basic Graph structure
f#classes

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

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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; }
end

type ('a, #graph_edge<'a>) graph_base = class
 val nodes : 'a array
 val edges : #graph_edge<'a> array
end

where graph_edge holds references to the from and to object and derived classes of graph_base must define a class for the nodes and a class based on graph_edge for the edges. However I am getting a syntax error on the declaration of the second type parameter to graph_base when I add the pound sign to indicate that classes based on graph_edge are acceptable. What am I doing wrong here?

David

.

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

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type ('a, 'b)graph_base when 'b :> graph_edge<'a> = class
  val nodes : 'a array
  val edges : 'b array
end

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

By on 5/8/2007 7:49 AM ()

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

By on 5/9/2007 8:42 AM ()

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