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# parvars

This folder contains an alternative version of the `parametric` example
that uses _parameter variables_ instead of the `parameter/2` and `this/1`
built-in methods to access entity parameters.

Print Logtalk, Prolog backend, and kernel versions (if running as a notebook):

```logtalk
%versions
```

Start by loading the example:

```logtalk
logtalk_load(parvars(loader)).
```

Some queries using the list parametric object:

```logtalk
%%table
[1, 2, 3]::member(X).
```

<!--
X = 1 ;
X = 2 ;
X = 3 ;
false.
-->

```logtalk
[1, 2, 3]::last(X).
```

<!--
X = 3.
-->

```logtalk
[1, 2, 3]::nextto(2, Y).
```

<!--
Y = 3.
-->

```logtalk
\+ []::member(X).
```

<!--
true.
-->

Some queries using the time and date parametric objects:

```logtalk
date(Year, Month, Day)::today.
```

<!--
Year = 2000, Month = 8, Day = 15.
-->

```logtalk
date(Year, _, _)::today, \+ date(Year, _, _)::leap_year.
```

<!--
Year = 2002.
-->

```logtalk
time(Hours, Mins, Secs)::now.
```

<!--
Hours = 13, Mins = 52, Secs = 42.
-->

```logtalk
rectangle(W, H, X, Y)::init, rectangle(W, H, X, Y)::move(3, 4, NR), NR::position(X2, Y2).
```

<!--
W = 2, H = 1, X = 0, Y = 0, NR = rectangle(2, 1, 3, 4), X2 = 3, Y2 = 4.
-->

Some queries with parametric objects that define "setter"
methods that return updated object identifiers:

```logtalk
person(sally, 20)::grow_older(NewId).
```

<!--
NewId = person(sally, 21).
-->

```logtalk
employee(sally, 21, 1200)::give_raise(250, NewId).
```

<!--
NewId = employee(sally, 21, 1450).
-->

Some queries with parametric categories:

```logtalk
speech(winter, wedding)::advice.
```

<!--
Clothes: [pants, sleeves, heavy]
Speech:  [happy, jokes]

true.
-->
