[m-users.] Returning a predicate from a function

[Sean Charles (emacstheviking)]

Literally cutting and pasting and compile I get:

tests.m:028: In clause for function `f_ln'/1:
tests.m:028:   error: undefined predicate `is'/2.

Which is not your error, have you pasted the right code?



> On 22 Dec 2022, at 11:52, Mark Clements <mark.clements at ki.se> wrote:
> 
> Dear Mercury users,
> 
> Is there any way to write a function that takes a predicate and returns a transformed predicate without (a) naming a new predicate or (b) using a lambda? In practice, I would like to move the lambda to a function. However, the following code gives an error: "Cannot unify two terms of type `(pred float)'". Can I assume that this is outside of Mercury's mode system?
> 
> Sincerely, Mark.
> 
> -----
> 
> :- module tests.
> 
> :- interface.
> :- import_module io.
> :- pred main(io::di, io::uo) is det.
> 
> :- implementation.
> :- import_module float, ranges, solutions.
> :- use_module math.
> 
> main(!IO) :-
>     test1(!IO), % OK
>     test2(!IO). % Does not compile
> 
> :- pred test1(io::di, io::uo) is det.
> test1(!IO) :- 
>     Data = (pred(Y::out) is nondet :- nondet_member(I,range(1,5)), Y = float(I)), % some data
>     LnData = (pred(Y::out) is nondet :- Data(X), Y = math.ln(X)), % a transformation
>     aggregate(LnData, print_line, !IO).
> 
> :- pred test2(io::di, io::uo) is det.
> test2(!IO) :- 
>     Data = (pred(Y::out) is nondet :- nondet_member(I,range(1,5)), Y = float(I)), % some data
>     LnData = f_ln(Data), %% This transformation does not compile:(
>     aggregate(LnData, print_line, !IO).
> 
> :- func f_ln(pred(float)::(pred(out) is nondet)) = (pred(float)::(pred(out) is nondet)).
> f_ln(Pred) = (pred(Y::out) is nondet :- Pred(X), Y is math.ln(X)).
> 
> 
> 
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