[m-users.] Correct use of solutions.

[Sean Charles (emacstheviking)]

HA! Yes, a poor choice of word, I meant trivial as in I knew how to do it and not spend half an hour in an argument with a compiler.

I've said it before, I have long wanted to write my transpiler, I have working versions in C, Haskell and SWI Prolog but ultimately, the long term robustness I seek and the worries of not having to deal with memory allocation mean that, despite it feeling like doing three rounds with Mike Tyson at times, I am determined to complete the mission!

As an untutored amateur, I absolutely appreciate what the compiler does for me in terms of making me think about what *I* am trying to do. I have barely used any of the advanced features such as high-order functions and type-classing with subtypes etc. I use those when I think they might help.

Sometimes I sit and think for a day or two before resuming coding of my project. I love the Mercury *makes me think* about what I am doing, and I am beginning to understand its internal dialogue too, I think if it was written in plainer more easily assimilated language that would have helped; as I say, I didn't do a CS degree, so a lot of the terminology I have to Wikipedia and go down a rabbit hole for a while, not that I mind! Learning and using Mercury this last couple of years has helped me be a better developer in Python and React by the simple transfer of techniques to other languages, not always straightforward, but it always ends well.

For me, I think that the software industry would be in much better shape if everybody had to use Mercury! I have to maintain a React application as part of my day job, it's a relatively simple UI, it has close to 200,000 files in node_modules. Unreal. And probably unsafe somewhere.

Thanks.
Sean.


> On 29 Jul 2023, at 12:09, Mark Brown <mark at mercurylang.org> wrote:
> 
> On Sat, Jul 29, 2023 at 6:27 PM Sean Charles (emacstheviking)
> <objitsu at gmail.com> wrote:
>> 
>> I've been trying to produce a simple list of strings to output as the response to a command line argument request to list the supported targets of my transpiler, given I have a type and a predicate to return the printable string for the language I thought solutions/2 was my answer, but not so far! Again, it's a mixture of the terminology to my untutored brain and the lack of any really clear guiding examples, in Prolog this stuff is trivial!
> 
> In Prolog this stuff is usually *concise*, but I wouldn't call it
> trivial. To verify correctness of bagof, for example, you need to
> reason about binding patterns and determinism. In the case of
> solutions/2, Mercury is doing a lot of the work for you, as well as
> for people reading your code, but like a lot of things in Mercury the
> benefits come at the cost of restricting yourself to mode-correct code
> and declaring what you are doing.
> 
> Whether verifiability is more important than brevity depends on your
> project, of course. Personally I find thinking to be more of a
> bottleneck than reading or typing, so it's usually a higher priority.
> But maybe that is just me!
> 
> Cheers,
> Mark
> 
>> 
>> 
>>    % Does -T / --targets.
>>    %
>> :- type supported_target
>>    --->    language_c
>>    ;       language_python
>>    ;       language_pythont
>>    ;       language_js.
>> 
>> :- pred show_targets(io::di, io::uo) is det.
>> 
>> 300:show_targets(!IO) :-
>> 301:    io.format("Available target languages:", [], !IO),
>> 302:    solutions(
>> 303:       (pred(A::out) is nondet :-
>> 304:            target_name(_, A)
>> 305:        ),
>> 306:       Targets
>> 307:    ),
>> 308:    io.print_line(Targets, !IO).
>> 
>> 
>> 
>> :- pred target_name(supported_target, string).
>> :- mode target_name(in, out) is det.
>> 
>> target_name(language_c, "Vanilla C").
>> target_name(language_python, "Vanilla Python").
>> target_name(language_pythont, "Typed Python").
>> target_name(language_js, "Vanilla JavaScript").
>> 
>> gives me the follwiong output:
>> 
>> command_line.m:308: In clause for `show_targets(di, uo)':
>> command_line.m:308:   mode error in conjunction. The next 3 error messages
>> command_line.m:308:   indicate possible causes of this error.
>> command_line.m:308:
>> command_line.m:303:   In clause for `show_targets(di, uo)':
>> command_line.m:303:   mode error in conjunction. The next 2 error messages
>> command_line.m:303:   indicate possible causes of this error.
>> command_line.m:303:
>> command_line.m:304:   In clause for `show_targets(di, uo)':
>> command_line.m:304:   in argument 1 of call to predicate
>> command_line.m:304:   `command_line.target_name'/2:
>> command_line.m:304:   mode error: variable `V_5' has instantiatedness `free',
>> command_line.m:304:   expected instantiatedness was `ground'.
>> command_line.m:303:
>> command_line.m:303:   In clause for `show_targets(di, uo)':
>> command_line.m:303:   in argument 1 of clause head:
>> command_line.m:303:   mode error in unification of `LambdaHeadVar__1' and `A'.
>> command_line.m:303:   Variable `LambdaHeadVar__1' has instantiatedness `free',
>> command_line.m:303:   variable `A' has instantiatedness `free'.
>> command_line.m:308:
>> command_line.m:302:   In clause for `show_targets(di, uo)':
>> command_line.m:302:   in call to predicate `solutions.solutions'/2:
>> command_line.m:302:   mode error: arguments `V_12, Targets' have the following
>> command_line.m:302:   insts:
>> command_line.m:302:     free,
>> command_line.m:302:     free
>> command_line.m:302:   which does not match any of the modes for predicate
>> command_line.m:302:   `solutions.solutions'/2.
>> command_line.m:302:   The first argument `V_12' has inst `free', which does not
>> command_line.m:302:   match any of those modes.
>> command_line.m:308:
>> command_line.m:308:   In clause for `show_targets(di, uo)':
>> command_line.m:308:   in argument 1 of call to predicate `io.print_line'/3:
>> command_line.m:308:   mode error: variable `Targets' has instantiatedness
>> command_line.m:308:   `free',
>> command_line.m:308:   expected instantiatedness was `ground'.
>> 
>> 
>> I've read the mercury crash course site with it's 'rows()' but so far drawn a blank...just how simple is it to get a list of possible solutions so that I may print a list of strings representing supported languages?
>> 
>> Thanks.
>> 
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