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# metapredicates

This example shows the use of meta-predicates in Logtalk. Meta-predicates are
predicates whose head includes arguments that will be called as goals in the
body of the predicate definition.

This example defines the following objects:

- `company`  
	usage example of the `map_reduce/5` meta-predicate

- `fibonacci`  
	example of calculating Fibonacci numbers using the `fold_left/4`
	meta-predicate

- `sort(_)`  
	this is a parametric object containing a method that implements the
	quicksort sorting algorithm; the parameter is interpreted as the type
	of the elements being sorted

- `tracer`  
	this object implements a meta-predicate that is used by `sort(_)` to 
	trace the sorting algorithm steps

- `metapreds`, `descendant`, and `test`  
	objects used for illustrating the use of closures as meta-arguments

- `predicates`  
	object defining some predicates for testing meta-predicates defined 
	in the Logtalk library

- `grammar`  
	object illustrating using `phrase/2` as a meta-predicate meta-argument

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

```logtalk
%versions
```

Start by loading the example and the required library files:

```logtalk
logtalk_load(metapredicates(loader)).
```

Try the simple meta-predicate definitions from the "goals.lgt" file:

```logtalk
simple_client::test_whatever.
```

<!--
true.
-->

```logtalk
simple_client::test_whatever_all.
```

<!--
Hello world!

true.
-->

Alternatively:

```logtalk
simple_client_alt::test_whatever.
```

<!--
true.
-->

```logtalk
simple_client_alt::test_whatever_all.
```

<!--
Hello world!

true.
-->

Note that _user_ is a pseudo-object representing the Prolog database;
therefore, the integer comparisons are performed using the standard
Prolog built-in predicates:

```logtalk
sort(user)::sort([3, 1, 4, 2, 9], Sorted).
```

<!--
call: partition([1,4,2,9],3,_358,_359)
exit: partition([1,4,2,9],3,[1,2],[4,9])
call: sort([1,2],_740)
call: partition([2],1,_967,_968)
exit: partition([2],1,[],[2])
call: sort([],_1300)
exit: sort([],[])
call: sort([2],_1539)
call: partition([],2,_1765,_1766)
exit: partition([],2,[],[])
call: sort([],_2093)
exit: sort([],[])
call: sort([],_2332)
exit: sort([],[])
exit: sort([2],[2])
exit: sort([1,2],[1,2])
call: sort([4,9],_2831)
call: partition([9],4,_3058,_3059)
exit: partition([9],4,[],[9])
call: sort([],_3391)
exit: sort([],[])
call: sort([9],_3630)
call: partition([],9,_3856,_3857)
exit: partition([],9,[],[])
call: sort([],_4184)
exit: sort([],[])
call: sort([],_4423)
exit: sort([],[])
exit: sort([9],[9])
exit: sort([4,9],[4,9])

Sorted = [1,2,3,4,9].
-->

Test the `setof/3` wrapper:

```logtalk
wrappers_client::p(L).
```

<!--
L = [1, 2, 3].
-->

```logtalk
wrappers_client::q(L).
```

<!--
L = [1, 2, 3].
-->

Call the meta-predicate `apply/2` directly:

```logtalk
metapreds::test_this.
```

<!--
1, metapreds

true.
-->

Send an `apply/2` message to self:

```logtalk
descendant::test_self.
```

<!--
2, descendant

true.
-->

Send an `apply/2` message from another object:

```logtalk
test::test_obj.
```

<!--
3, test

true.
-->

Use the `partition/4` meta-predicate with a predicate defined in the
pseudo-object `user`:

```logtalk
meta::partition(even_integer, [1,2,3,4,5], Included, Excluded).
```

<!--
Included = [2, 4], Excluded = [1, 3, 5].
-->

Get a visual illustration of the differences between left and right folds:

```logtalk
folds::left(Left).
```

<!--
Left = '(((((((((0+1)+2)+3)+4)+5)+6)+7)+8)+9)'.
-->

```logtalk
folds::right(Right).
```

<!--
Right = '(1+(2+(3+(4+(5+(6+(7+(8+(9+0)))))))))'.
-->

Walk a list using left and right folds (note the lambda parameters order):

```logtalk
meta::fold_left([Y, X, [X|Y]]>>true, [], [1,2,3,4,5,6,7,8,9], R).
```

<!--
R = [9, 8, 7, 6, 5, 4, 3, 2, 1]
-->

```logtalk
meta::fold_right([X, Y, [X|Y]]>>true, [], [1,2,3,4,5,6,7,8,9], R).
```

<!--
R = [1, 2, 3, 4, 5, 6, 7, 8, 9].
-->

We can compose left folds:

```logtalk
meta::fold_left([X,Y,Z]>>(Z is X + Y), 0, [1,2,3], Sum).
```

<!--
Sum = 6.
-->

```logtalk
meta::fold_left(meta::fold_left([X,Y,Z]>>(Z is X + Y)), 0, [[1,2,3],[4,5,6]], Sum).
```

<!--
Sum = 21.
-->

Use the `fold_left/4` meta-predicate with a predicate defined in the
pseudo-object _user_:

```logtalk
meta::fold_left(sum_squares, 0, [1,2,3], Result).
```

<!--
Result = 34.
-->

Use the `fold_left/4` and `fold_right/4` meta-predicates with the `atom_concat/3`
Prolog built-in predicate:

```logtalk
meta::fold_left(atom_concat, 'PREFIX', [abc,def,ghi], Result).
```

<!--
Result = 'PREFIXabcdefghi'.
-->

```logtalk
meta::fold_right(atom_concat, 'SUFIX', [abc,def,ghi], Result).
```

<!--
Result = abcdefghiSUFIX.
-->

Use the `fold_left/4` meta-predicate with predicates defined in the object
`predicates`:

```logtalk
meta::fold_left(predicates::sum, 0, [1,2,3,4,5], Result).
```

<!--
Result = 15.
-->

```logtalk
meta::fold_left(predicates::product, 1, [1,2,3,4,5], Result).
```

<!--
Result = 120.
-->

```logtalk
meta::fold_left(predicates::tuple, (0,0), [(1,2), (3,4), (6,4)], Result).
```

<!--
Result = (10, 10).
-->

Alternatives to `fold_left/4` and `fold_right/4` when there isn't an initial value:

```logtalk
meta::fold_left_1([X,Y,Z]>>(Z is X+Y), [1,2,3,4,5], R).
```

<!--
R = 15.
-->

```logtalk
meta::fold_right_1([X,Y,Z]>>(Z is X-Y), [1,2,3,4,5], R).
```

<!--
R = 3.
-->

```logtalk
meta::fold_right_1([X,Y,Z]>>(Z is X*Y), [1,2,3,4,5], R).
```

<!--
R = 120.
-->

Use the `scan_left/4` and `scan_right/4` meta-predicates with predicates
defined in the object `predicates`:

```logtalk
meta::scan_left(sum_squares, 0, [1,2,3], Result).
```

<!--
Result = [0, 1, 5, 34].
-->

```logtalk
meta::scan_right(predicates::sum, 5, [1,2,3,4], Result).
```

<!--
Result = [15, 14, 12, 9, 5].
-->

Compute a sequence of factorial numbers using a scan of natural numbers
using multiplication:

```logtalk
meta::scan_left([X,Y,Z]>>(Z is X*Y), 1, [1,2,3,4,5,6], Result).
```

<!--
Result = [1, 1, 2, 6, 24, 120, 720].
-->

Use the `map/2-3` meta-predicates with some Prolog built-in predicates.

Check that all list elements are integers:

```logtalk
meta::map(integer, [1,2,3,4,5]).
```

<!--
true.
-->

Map characters to character codes:

```logtalk
meta::map(char_code, [a,b,c,d,e], Codes).
```

<!--
Codes = [97, 98, 99, 100, 101].
-->

Use the `fold_left/4` meta-predicate to calculate Fibonacci numbers:

```logtalk
fibonacci::nth(10, Fib).
```

<!--
Fib = 55.
-->

The first six Fibonacci numbers:

```logtalk
%%table
integer::between(0, 5, Nth), fibonacci::nth(Nth, Fib).
```

<!--
Fib = 0, Nth = 0 ? ;
Fib = 1, Nth = 1 ? ;
Fib = 1, Nth = 2 ? ;
Fib = 2, Nth = 3 ? ;
Fib = 3, Nth = 4 ? ;
Fib = 5, Nth = 5 ? 
...
-->

Use the `map_reduce/5` meta-predicate to give bad news to a company employees:

```logtalk
company::(company(C1), get_salary(company(C1),S1)).
```

<!--
C1 = [topdept(name('Human Resources'),manager(name('Lisa'),salary(123456)),[]),topdept(name('Development'),manager(name('Anders'),salary(43210)),[subdept(name('Visual Basic'),manager(name('Amanda'),salary(8888)),[]),subdept(name('Visual C#'),manager(name('Erik'),salary(4444)),[])])]
S1 = 179998
-->

```logtalk
company::(company(C1), cut_salary(company(C1), C2), get_salary(C2, S2)).
```

<!--
C1 = [topdept(name('Human Resources'),manager(name('Lisa'),salary(123456)),[]),topdept(name('Development'),manager(name('Anders'),salary(43210)),[subdept(name('Visual Basic'),manager(name('Amanda'),salary(8888)),[]),subdept(name('Visual C#'),manager(name('Erik'),salary(4444)),[])])]
C2 = company([topdept(name('Human Resources'),manager(name('Lisa'),salary(61728)),[]),topdept(name('Development'),manager(name('Anders'),salary(21605)),[subdept(name('Visual Basic'),manager(name('Amanda'),salary(4444)),[]),subdept(name('Visual C#'),manager(name('Erik'),salary(2222)),[])])])
S2 = 89999
-->
