[m-users.] Higher-order predicate issue

[Peter Wang]

On Fri, 01 Oct 2021 21:27:40 +0100 "Sean Charles (emacstheviking)" <objitsu at gmail.com> wrote:
> Hi,
> 
> I am trying to pass around a predicate to execute at a later point in my translation process:
> 
> :- type renderer == (pred(instruction, lstring, rop_error, tcon, tcon)).
> :- inst renderer == (pred(in, in, out, in, out) is det).
> 
> then to set the renderer:
> 
>     Renderer = target_c.render
> 
> then I have a work in progress evaluation predicate:
> 
> :- pred eval(renderer::in, location::in, instruction::in, lsnode::in,
>     renderout::out, tcon::in, tcon::out) is semidet.

The mode 'in' is defined as 'ground >> ground',
so the pred-mode declaration declares that the first argument has
initial inst 'ground' and final inst 'ground'.

> 
> eval(Render, Pos, defvar, Args, Result, !T) :-
>     trace[io(!Dbg), runtime(env("FELT_TRN"))]
>     (sdump($pred, $line, Args, !Dbg)),
> 
>     ( if Args = [tk(Pos, VarName)] then
>             %
>             % this it not working well
>             % ...what can be done to make it better ?
>             %
> 348     Render(defvar, lstring(Pos, VarName), CodeM, !T),

Renderer has inst 'ground' here, meaning it has no higher-order inst
information, so it cannot be called. (It is possible to call a
higher-order *function* term without higher-order inst information,
but that is a special case.)

You need to change the mode of the first argument from 'in' to
'in(renderer)', which expands to 'renderer >> renderer',
where 'renderer' refers to the inst you have defined earlier.
There is no implied connection between the type name 'renderer'
and the inst name 'renderer'.

Peter