[mercury-users] Strange Error Message

[Gustavo A. Ospina]

Hello,

I got an error message when compiling this function:

:- pred print_pred_call(io:output_stream, log_pred, io:state, io:state).
:- mode print_pred_call(in, in, di, uo) is det.

print_pred_call(Stdout, Pr) -->
(    {Pr = cut},
   io:print(Stdout, '!')
;
   {Pr = unif(Var, Expr)},
   io:write_string(Stdout, Var),
   io:write_string(Stdout, " = "),
   print_expr(Stdout, Expr)
;
   {Pr = pred_call(P, Exprs)},
   io:write_string(Stdout, P),
   io:print(Stdout, '('),
   print_ls_expr(Stdout, Exprs),
   io:print(Stdout, ')')
).

This function is on the implementation part. 'log_pred' is a type 
imported from another module, defined as:

:- type log_pred --->
       cut
   ;   unif(string, log_expr)
   ;   pred_call(string, list(log_expr)).

The error is the following (with the -E flag):

xxx.m:117: In clause for predicate `lpid_to_prolog:print_pred_call/4':
xxx.m:117:   error: ambiguous overloading causes type ambiguity.
xxx.m:117:   Possible type assignments include:
xxx.m:117: V_23 :: pred(T, T) or (pred)
   You will need to add an explicit type qualification to resolve the
   type ambiguity.
   The way to add an explicit type qualification
   is to insert a call to a dummy predicate whose `:- pred'
   declaration specifies the appropriate argument types.

Number line is the corresponding to the head of the clause print_pred_call.

The error is not clear to me: what variable is really the one having 
conflicts with types?

I tried qualifying the variable Pr with its type, log_pred (using the 
'with type' expression), and making explicit the module prefixes for the 
type constructors in the switch, but it didn't work.

Maybe am I using an "ambiguous" name for some type constructor in Pr? I 
really don't know.

I'll thank you for any help.

+ Gustavo +
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