[m-users.] Higher-order predicate issue

[Sean Charles (emacstheviking)]

Thank you Zoltan and Peter then!

After a little more head scratching and using my go-to help tool, the mmc-doc sources, I have managed to work it out and I now have the following:

 64 :- type renderer == (pred(render_inst, rop_error, tcon, tcon)).
 65 :- mode renderer == (pred(in, out, in, out) is det).
:
106         Renderer = get_target_renderer(Target),
:
155 :- func get_target_renderer(string::in) = (renderer::out(renderer)).
156 :- inst renderer == (pred(in, out, in, out) is det).
157
158 get_target_renderer(T) = R :-
159     LCTarget = string.to_lower(T),
160     ( if LCTarget = "php" then
161         R = target_c.render
162     else
163         R = target_c.render
164     ).

And it all works beautifully, thank you both for your guidance and assitance. I am not 100% sure I totally understand what’s going on but using mmc-doc’s definition of:

:- type maybeio == (pred(maybe(string), io, io)).
:- inst maybeio == (pred(out, di, uo) is det).
:- func (maybeio::in(maybeio)) // (maybeio::in(maybeio))
    = (maybeio::out(maybeio)).

I managed to work it out and then referred back to section 4.1 on Modes and section 8 Higher-order programming for some closure on the matter, at leat for now. I will spend some more time on it later but The Boss she now insists I have to mow the lawn front and back. Meh.

Thanks as always,
Sean.


> On 2 Oct 2021, at 22:25, Zoltan Somogyi <zoltan.somogyi at runbox.com> wrote:
> 
> 
> 2021-10-03 07:11 GMT+11:00 "Sean Charles (emacstheviking)" <objitsu at gmail.com>:
>> :- func get_target_renderer(string::in) = (renderer::out).
> 
> This is mirror image of the bug Peter pointed out yesterday:
> the result of this function should have mode "out(renderer)"
> instead of just "out". Declaring the result to have mode "out"
> also throws away the mode information about the higher order
> result, so it is not surprising that you get mode errors when
> that higher order mode information is needed but not present,
> i.e. what you call Renderer.
> 
> Zoltan.

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