[m-users.] IO argument clobbering

[Richard O'Keefe]

Two points of pedantry.
Mercury:  :- type callback == ... is NOT like a preprocessor macro in C.
It's much closer to a C 'typedef'.
English:  the past participle of 'to grok' is 'grokked', not 'groked'.
The Groke https://en.wikipedia.org/wiki/The_Groke is not involved.
https://www.oxfordlearnersdictionaries.com/definition/english/grok

I had a bit of trouble getting the hang of higher order insts myself.

On Tue, 18 Jul 2023 at 21:39, Volker Wysk <post at volker-wysk.de> wrote:

> Am Dienstag, dem 18.07.2023 um 02:09 +1000 schrieb Zoltan Somogyi:
> > On 2023-07-17 17:43 +02:00 CEST, "Volker Wysk" <post at volker-wysk.de>
> wrote:
> > > Am Montag, dem 17.07.2023 um 17:39 +0200 schrieb Volker Wysk:
> > > > But the "in(callback)" inst isn't ground. It's a higher order inst.
> Only
> > > > the "in" with zero arguments is an abbreviation for "in(ground)".
> > >
> > > Hmmm... Looks like I was confused about what "ground" means.
> >
> > In different contexts, "ground" means several related and similar
> > but nevertheless slightly different things. Which meaning of "ground"
> people
> > mean when they write that term is *usually* clear from the context, but
> not always.
> >
> > The standard meaning in logic programming theory is simply "a term that
> > contains no variables", and all uses of "ground" include this meaning.
> But Mercury
> > goes beyond standard logic programming theory by having types,
> > including higher order types. When you write "ground" in a Mercury
> program,
> > that occurrence of "ground" describes a term that contains
> > no variables AND whose type is either not a higher order type,
> > or is a higher order function type with the standard function mode
> > (which is: all arguments are input, the return value is output).
> > It cannot be a higher order predicate type, or a higher order function
> > type with a nonstandard mode, because for those, you need to tell
> > the compiler the modes of their arguments. If you don't give the compiler
> > this info, you get the error message that started this thread.
> >
> > There is a similar issue with respect to uniqueness. The inst "ground"
> > describes a reference to a term that contains no variables, and is not
> > the only reference to that term. If you want to describe a reference
> > to a term that contains no variables but it IS the only reference
> > to that term, what you write is "unique".
> >
> > Internally in the Mercury compiler, the representation of ground
> > insts is parameterised by the absence/presence of both uniqueness
> > and higher order inst information. The "ground" inst in Mercury programs
> > corrresponds to the absence of both of these kinds of information.
> > And yet the compiler's name even for insts with one or both
> > of these kinds of information is still "ground". So you see, even
> > the Mercury language and the Mercury compiler disagree on
> > which variant meaning should be called "ground" :-)
>
> Okay, now I've groked what a "ground term" and a "ground inst" is in
> Mercury. Thank you for the explanation.
>
> Regards,
> Volker
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