[dcgreenwood]

I am learning prolog, going though the fundamentals, but I cannot find any description of what the [out] parameter is that I see in some of the example predicates.

[Harrison Pratt]

[out] indicates that a value is returned from the predicate.

For example:

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class predicates
    demoPred : (integer Input, string Output [out]).
clauses
    demoPred(Input, OutPut) :-
        if Input < 0 then
            OutPut = "Negative"
        elseif Input = 0 then
            OutPut = "Zero"
        else
            OutPut = "Positive"
        end if.

[dcgreenwood]

Thanks. Since most examples in the fundementals don't include it and allow a term to be either input or output, I assume it is and optional declaration.

[Harrison Pratt]

The default specification for predicate definitions is [in], so you don't need to specify that a parameter is an input.

You may be surprised to see that the IDE will change a valid definition like this:

Code: Select all

demoPred : (integer Input , string Output) (i,o).

to this preferred form:

Code: Select all

demoPred : (integer Input, string Output [out]).

The IDE does a nice job of standardizing code format. I would recommend you have that turned on in the Project Settings > Build Options. It makes it easier to compare your code to that of others or your own earlier code.