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Value Recursion
f#

I am running into an issue with value recursion I can't seem to solve. I want to create a graph where every node has a list to its neighboring nodes. I was thinking about a definition like this:

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type node =
    { Name: string;
      Nb: node list}

let rec (A:node) = {Name="A"; Nb=[B;C]}
and (B:node) = {Name="B"; Nb=[A;C]}
and (C:node) = {Name="C"; Nb=[A;B]}

However, when I get a compile time error saying:

Recursive values may not appear directly as a construction of the type 'List' within a recursive binding except in the assembly where that type is defined.

Any suggestions on how to get around this issue?

Thanks, Jurgen

.

You might be interested by this manual section: [link:research.microsoft.com]

Your example worked in OCaml, but recursion need to be delayed in F#. One solution is:

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type node =
    { Name: string;
      Nb: (node list) lazy}

let rec (A:node) = {Name="A"; Nb=lazy [B;C]}
and (B:node) = {Name="B"; Nb=lazy [A;C]}
and (C:node) = {Name="C"; Nb=lazy [A;B]}

And then, to get neighbours: Lazy.force A.Nb or A.Nb.Force

I don't know if we can get something prettier.

Laurent.

By on 3/5/2007 1:10 AM ()

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