[mercury-users] Hash_table initialization

[Goncalo Jorge Coelho e Silva]

Hi,

I had a problem while declaring a predicate signature
with an hash_table as an argument.

I think it has to do with a wrong hash_table argument fingerprint 
declaration, in pred for lookup_label/5...

should it be 'hash_table(string, int)'? ... it seems not...
what then?

I tried this out:



:- module inter.
:- interface.
:- import_module io.

:- pred main(io__state::di, io__state::uo) is det.

:- pred lookup_label(hash_table(string, int), string, int, io__state, io__state).   
:- mode lookup_label(in, in, out, in, out) is det.    

:- implementation.
:- import_module hash_table.


lookup_label(HashTable, Label, Index) --> 
 	{ 
        Index = HashTable^elem(Label) 
 	}. 

% same without DCGs
%lookup_label(HashTable, Label, Index, IO, IO):- 
%       Index = HashTable^elem(Label).


main(IO0,IO) :-
     		% create an empty hash table
		HashTable0 = new_default(string_double_hash),

		% insert a key and value into the hash table
		%HashTable1 = det_insert(HashTable0, "some key", 42),
 		HashTable1 = ( HashTable0 ^ elem("some key") := 42),

                % lookup a key in the hash table
		N = HashTable1^elem("some key"),

		% print the result
		print("N = ", IO0, IO1),
		print(N, IO1, IO2), 
		nl(IO2, IO). 


Compilation result was: 

inter.m:018: In clause for `lookup_label(in, in, out, in, out)':
inter.m:018:   mode error in conjunction. The next 2 error messages
inter.m:018:   indicate possible causes of this error.
inter.m:018: In clause for `lookup_label(in, in, out, in, out)':
inter.m:018:   in argument 2 of call to function `hash_table:elem/2':
inter.m:018:   mode error: variable `HashTable' has instantiatedness
`ground',
inter.m:018:   expected instantiatedness was `(hash_table:hash_table)'.
inter.m:016: In clause for `lookup_label(in, in, out, in, out)':
inter.m:016:   in argument 3 of clause head:
inter.m:016:   mode error in unification of `HeadVar__3' and `Index'.
inter.m:016:   Variable `HeadVar__3' has instantiatedness `free',
inter.m:016:   variable `Index' has instantiatedness `free'.
make: *** [run] Error 1


 Tips on what's wrong would be appreciated :)


Thanks again.

Cheers,
Goncalo


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