[mercury-users] define predicate with currying?

[Julien Fischer]

Hi,

On Thu, 5 Jan 2012, Michael Hendricks wrote:

> I have the following simple program:
>
>    main(!IO) :-
>        hello([s("World")], !IO),
>        hello([s("Mercury")], !IO).
>
>    :- pred hello(list(string.poly_type)::in, io::di, io::uo).
>    hello(Xs,!IO) :- format("Hello, %s!\n",Xs,!IO).
>
> Is it possible to define 'hello' by currying 'format'?  In Haskell,
> I'd do something like
>
>    hello :: String -> IO ()
>    hello = printf "Hello, %s!\n"
>
> The best I could come up with in Mercury was
>
>    main(!IO) :-
>        Hello = format("Hello, %s!\n"),
>        Hello([s("World")], !IO),
>        Hello([s("Mercury")], !IO).
>
> Of course, that only defines 'Hello' locally.  I was hoping for a
> global definition.

In general, function composition of that style is bit less elegant in
Mercury than it is in Haskell.  One reason for this is, of course, that
not everything in Mercury is a function, in particular io.format/4 is a
predicate and the Mercury compiler needs to be able to distinguish
between the two.

The closest I think we can get to the above Haskell in Mercury is
a function that returns a curried predicate, something like the
following:

     :- func hello =
         pred(list(string.poly_type), io, io)::out(pred(in, di, uo) is det)).

     hello = format("Hello, %s\n").

     main(!IO) :-
       (hello)([s("World")], !IO),
       (hello)([s("Mercury")], !IO).

(Note that you need to parenthize a call to a zero-arity function in
Mercury.)

Julien.
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