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# eval_to_array(?Collection, ?Result)

Equate a "collection" expression with an array

Collection
  A term to be interpreted as a collection
Result
  The collection as array

## Description

This is a constraint establishing equality between a collection-valued expression and an array containing the same elements as the collection. Evaluation delays if arguments are insufficiently instantiated.

A "collection" can be either of the following data terms:

List
  The list is converted into an array, as with array_list/2.
Array
  The array is returned unchanged.

In addition, the following "collection expressions" are allowed:

Array[...]
  Subscript-reference: Extract an element or sub-array from Array. If a single array element is extracted, this element must itself be a collection (array or list).
CollectionExpr1>>CollectionExpr2
  The result is the concatenation of the two collections.
concat(CollectionExpr)
  If the collection is nested (at least 2-dimensional), the top level of the structure is removed and the result is the concatenation of all its elements (which must be collections themselves).
eval(X)
  If X is a list, array or collection expression, then eval(X) is equivalent to X alone. If X is uninstantiated, then eval(X) indicates that X might be instantiated to any collection expression term. Without the eval/1 wrapper, a free X is interpreted as an uninstantiated list.

### Modes and Determinism

- eval_to_array(?, ?) is semidet

### Fail Conditions

Collection is not a collection expression, or Result does not unify with the evaluation result

## Examples

       ?- List=[a,b,c,d], eval_to_array(List, Result).
       Result = [](a, b, c, d)

       ?- Arr=[](a,b,c,d), eval_to_array(Arr, Result).
       Result = [](a, b, c, d)

       ?- Arr=[](a,b,c,d), eval_to_array(Arr[2..3], Result).
       Result = [](b, c)

       ?- Mat=[]([](a,b,c),[](d,e,f)), eval_to_array(Mat, Result).
       Result = []([](a, b, c), [](d, e, f))

       ?- Mat=[]([](a,b,c),[](d,e,f)), eval_to_array(concat(Mat), Result).
       Result = [](a, b, c, d, e, f)

       ?- Mat=[]([](a,b,c),[](d,e,f)), eval_to_array(Mat[1], Result).
       Result = [](a, b, c)

       ?- Mat=[]([](a,b,c),[](d,e,f)), eval_to_array(Mat[1,*], Result).
       Result = [](a, b, c)

       ?- Mat=[]([](a,b,c),[](d,e,f)), eval_to_array(Mat[*,2], Result).
       Result = [](b, e)

       ?- Mat=[]([](a,b,c),[](d,e,f)), eval_to_array(Mat[1..2,2], Result).
       Result = [](b, e)

       ?- Mat=[]([](a,b,c),[](d,e,f)), eval_to_array(Mat[1..2,2..3], Result).
       Result = []([](b, c), [](e, f))

       ?- Mat=[]([](a,b,c),[](d,e,f)),
    			eval_to_array(concat(Mat[1..2,2..3]), Result).
       Result = [](b, c, e, f)

       ?- NL = [a,b,[c,d]], eval_to_array(NL, Result).
       Result = [](a, b, [c, d])

       ?- NL = [a,b,[](c,d)], eval_to_array(NL, Result).
       Result = [](a, b, [](c, d))

       ?- NA = [](a,b,[c,d]), eval_to_array(NA, Result).
       Result = [](a, b, [c, d])

       ?- NA = [](a,b,[c,d]), eval_to_array(NA[3], Result).
       Result = [](c, d)

       ?- Xs=[a,b], Yz=[](c,d), eval_to_array(Xs>>Yz, Result).
       Result = [](a, b, c, d)

       % Error cases where collections expected
       ?- eval_to_array(no_collection, Result).
       No (0.00s cpu)

       ?- eval_to_array(99, Result).
       No (0.00s cpu)

       ?- eval_to_array(concat([[1],2,[3]]), Result).
       No (0.00s cpu)

       % Cases with insufficient instantiation:

       ?- eval_to_array(Xs, R).
       R = R
       <delays until Xs (or R) instantiated>

       ?- eval_to_array([1|Xs], R).
       R = R
       <delays until Xs instantiated>

       ?- eval_to_array([1|Xs], R), Xs=[2].
       R = [](1,2)
       Yes

       ?- eval_to_array(Xs>>[3], R).
       R = R
       <delays until Xs instantiated>

       ?- eval_to_array(Xs>>[3], R), Xs=[1].
       R = [](1,2)
       Yes

       ?- eval_to_array([1]>>Ys, R).
       R = R
       <delays until Ys instantiated>

       ?- eval_to_array([1]>>Ys, R), Ys=[2].
       R = [](1,2)
       Yes

       ?- Mat=[]([](a,b,c),[](d,e,f)), eval_to_array(Mat[I], R).
       R = R
       <delays until I instantiated>

       % Delayed instantiation of expression
       ?- eval_to_array(eval(X), [](1,2,3)).
       X = X
       <delays until X instantiated>

       ?- eval_to_array(eval(X), Ys), X=[1]>>[2,3].
       Ys = [](1, 2, 3)

       ?- eval_to_array(eval(X), [](1,2,3)), X=[](1,2,3).
       Yes

       % Reverse direction
       ?- eval_to_array(Xs, [](1,2,3)).
       Xs = [1,2,3]
       Yes

## See Also

eval_to_complete_list / 2, eval_to_array / 2, eclipse_6 : collection_to_list / 2, lists : collection_to_list / 2, array_list / 2, array_concat / 3, lists : append / 3, is_list / 1, is_array / 1, subscript / 3