[m-users.] Passing field access functions to map?

[Jeremy W. Sherman]

Is there some way to reference field access functions as functions in
themselves?

They seem to work fine when applied directly, e.g. f(HasField), and
with record syntax, e.g. HasField^f, but I can't work out how to pass
either flavor of field function directly to a function like map or
fold, whether with or without currying.

As a workaround, I can manually declare a function to shuttle the args
in and out to the fully-applied field access functions, but I'd rather
not have to write this boilerplate.

The following is an example module (that does not compile)
demonstrating what I have tried so far. The example is also visible at
https://github.com/jeremy-w/adventofcode-2020/blob/main/src/ex_field_hofs.m
if that's more convenient. (The context I ran into this was when
working with a point(x,y,z) type in day17.m.)

==========
:- module ex_field_hofs.

:- interface.
:- type has_field ---> has_field(f :: int).

:- implementation.
:- import_module int.
:- import_module list.
:- import_module string.

%-----------------------------------------------------------------------------%
% ACCESS EXAMPLES
% Several attempts, all unsuccessful, to use `f` as a function in its own right.

% Example of attempting to use a field accessor as an argument to a
higher-order function.
%
% Triggers unification error when written directly as `f` - it seems
`f` is being interpreted as a `char`:
% type error in unification of argument and constant `f'.
% argument has type `((func ex_field_hofs.has_field) = int)',
% constant `f' has type `character'.
:- func access_example1(list(has_field)) = list(int).
access_example1(Hs) = map(f, Hs).

% Attempting to qualify it as ex_field_hofs.f complains:
% error: undefined symbol `ex_field_hofs.f'/0.
:- func access_example2(list(has_field)) = list(int).
access_example2(Hs) = map(ex_field_hofs.f, Hs).

% Attempting to include the ^ shown in "Field access examples"
(https://mercurylang.org/information/doc-release/mercury_ref/Field-access-examples.html#Field-access-examples)
fails as well:
% error: undefined symbol `^ f'/0
% Trying without the single quotes (not shown here) instead gives:
% error: undefined symbol `^'/1.
:- func access_example3(list(has_field)) = list(int).
access_example3(Hs) = map('^ f', Hs).

%-----------------------------------------------------------------------------%
% UPDATE EXAMPLE

% Example of attempting to use the field setter as an argument to a
higher-order function.
% Per
:- func update_example(has_field, list(int)) = list(has_field).
update_example(H, Ns) = map('f :='(H), Ns).
==========

Thanks,
--
Jeremy W. Sherman
https://jeremywsherman.com/