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

[Mark Clements]

Thank you everyone -- that has been very helpful. I admit that I was using an older version of Mercury (20.06.1). On that version, I *was* able to use either `=` or `is` for the mathematical equations. I have now installed the latest release.

 I admit that I was using an older version of Mercury (20.06.1). On that version, I *was* able to use either `=` or `is` for the mathematical equations. I have now installed the latest release.

Julien: that was the answer that I was hoping for:).

Tomas: your code is elegant. I understand that lists or arrays would be quite suitable for the given example -- but my motivating case is for doing bag aggregates on (nondet) relationships (see https://rosettacode.org/wiki/Merge_and_aggregate_datasets#Mercury).
[https://static.miraheze.org/rosettacodewiki/d/d3/RosettaCodeTitle.png]<https://rosettacode.org/wiki/Merge_and_aggregate_datasets#Mercury>
Merge and aggregate datasets - Rosetta Code<https://rosettacode.org/wiki/Merge_and_aggregate_datasets#Mercury>
Merge and aggregate datasets Task. Merge and aggregate two datasets as provided in .csv files into a new resulting dataset. Use the appropriate methods and data structures depending on the programming language.
rosettacode.org

With the recent interest in Verse (https://news.ycombinator.com/item?id=33946933), which combines nondeterminism and functional aspects, I hope there will be increased interest in Mercury.

Sincerely, Mark.

________________________________
From: Tomas By <tomas at basun.net>
Sent: 22 December 2022 16:13
To: Mark Clements <mark.clements at ki.se>
Cc: users at lists.mercurylang.org <users at lists.mercurylang.org>
Subject: Re: [m-users.] Returning a predicate from a function

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On Thu, 22 Dec 2022 12:52:52 +0100, Mark Clements wrote:
> 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)).


I'm not 100% sure what you mean for this to do. In particular I am not
sure what "aggregate" does (in Python I assume).

Some general points: maths is not nondet; the basic data structure is
a list. not some abstract generator as in some other languages that I
could mention; you do not always need all these declarations.

This seems to work:


|:- module test.
|
|:- interface.
|:- import_module io.
|:- pred main(io::di, io::uo) is det.
|
|:- implementation.
|:- import_module int, float, list.
|:- use_module math.
|
|main(!IO) :-
|  test1(!IO),
|  test2(!IO).
|
|:- pred test1(io::di, io::uo) is det.
|
|test1(!IO) :-
|  Data = map(float,1..5),
|  LnData = map(math.ln,Data),
|  write_list(LnData,"",print_line,!IO).
|
|:- pred test2(io::di, io::uo) is det.
|
|test2(!IO) :-
|  Data = map(float,1..5),
|  LnData = map(f_ln,Data),
|  write_list(LnData,"",print_line,!IO).
|
|:- func f_ln(float::in) = (float::out) is det.
|
|f_ln(F0) = F :- F = math.ln(F0).
|
|:- end_module test.


Is this what you meant?

/Tomas



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