[m-users.] Higher-order predicates and functions and equivalence types.

[Sean Charles (emacstheviking)]

Hi,

I have a type definition like this:

:- type tcon
    --->    tcon(
                options  :: felt_options,
                output   :: list(rop),
                warnings :: lserror,
                errors   :: lserror,
                render   :: pred(instruction::in, string::in, string::out) is semidet
                % ENV[] stack can go in here too
            ).

The `render` field, I wanted to be able to code this something like:

    render :: foo

where foo would have been defined as

:- type foo = (pred(instruction::in, string::in, string::out) is semidet).

because I also wanted to be able to declare predicates that take a `foo` but it seems that I can’t do that. I’ve read Chapter 8 — Higher-order programming but I find my lack of familiarity with the terminology once again leaving me stumped.

I guess what I am after is something like the classic ‘pointer to a function returning an int’ in C.

How would I approach such a thing in Mercury?

Thank you,
Sean

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