Re: eqtype and functors

On May 31, 2007, at 10:55 PM, brian wrote:

> Well I was trying to understand how to make the code operate as  
> intended.
> Obviously the purposed of the GRAPH_NODE sig is so that you can use  
> some other datatype as node info.

But then having a type definition doesn't make sense.  Either you  
want the node type
to be an abstract equality type, or you want it to be a concrete type.

>
> Upon fursther reflection, it seems like it should just use int's  
> all the time.
> Any associated data can simply be stored in an array indexed by the  
> node number.
>
> Do you (or anyone else) know of any good graph algorithm packages ?
>
> Or am I going to have to write one _again_ ? ;-)

Graph libraries are hard to design (Emden Gansner and I tried a  
couple of approaches,
but we were never happy with the tradeoffs).  The MLRISC toolkit has  
a fairly complete
graph library (see MLRISC/graphs) that you might find useful.

	- John


>
>
> Brian
>
> On May 31, 2007, at 7:26 AM, John Reppy wrote:
>
>> A better fix would be to just drop the "eq", since int is an eqtype
>> by definition.
>>
>> 	- John
>>
>> On May 30, 2007, at 10:39 PM, brian wrote:
>>
>>> Hi,
>>>
>>> I'm trying to use fgl (a functional graph library) and I ran across
>>> this:
>>>
>>> signature GRAPH_NODE =
>>> sig
>>>    eqtype node = int
>>> end
>>>
>>> sml throws an error on that statement.  I simply removed the "= int"
>>> to fix it.
>>>
>>> This change however causes another problem.  I've put the rest of it
>>> at the bottom of the e-mail because I think the short question is:
>>> what do I do to implement the intent of the original code ?
>>>
>>> Thanks
>>>
>>> Brian
>>>
>>>
>>> applic/lamtr.sml:128.18-128.50 Error: operator and operand don't
>>> agree [tycon mismatch]
>>>    operator domain: (int * Value.node_label list * (G.node *  
>>> G.node *
>>> 'Z) list
>>>                      -> (Value.node_label,'Z) G.graph)
>>>                     * (int * int * CombExpr.comb_term
>>>                        -> int * Value.node_label list
>>>                           * (G.node * G.node * 'Z) list)
>>>    operand:         (int * Value.node_label list * (G.node *  
>>> G.node *
>>> 'Z) list
>>>                      -> (Value.node_label,'Z) G.graph)
>>>                     * (int * int * CombExpr.comb_term
>>>                        -> int * Value.node_label list
>>>                           * (int * int * Value.edge_label) list)
>>>    in expression:
>>>      G.mkgr o UTuple.t23 o toEdgeList
>>>
>>> Here's the statement:
>>>
>>>    fun toGraph r l = (G.mkgr o UTuple.t23 o toEdgeList) (r, 
>>> 0,E.comb l)
>>>
>>> and it's in a functor:
>>>
>>> functor LambdaTrans (G:GRAPH) =
>>> struct
>>> ...
>>>
>>> So if I parse the error message correctly, it looks as though there
>>> is a problem in the the function composition.  I believe that the
>>> problem is toEdgeList is assuming that the node/node ID is an
>>> integer, when in fact it's actually a TBD type.
>>>
>>>
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