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3.7 List comprehension expressions

Lists of ground terms can also be denoted by a list compehension expression. Examples are:

[X**2::X in L & not X in [0,1]]
% Squares of numbers other than 0,1 in nums list L

[C :: exists A child_of(C,peter) & age_of(C,A) & A<18]
% Non-adult children of peter in order found

The general form of a list comprehension is:

[Expression :: exists VarSequence SimpleConjunction]

where the exists VarSequence is optional.

There are constraints on the variables that can be used in such a comprehension. Each variable in Expression must either appear in SimpleConjunction and be such that it will be given a ground value by some call in the conjunction, or it must appear before the comprehension expression in a query or rule and will have been given a ground value. Every variable in SimpleConjunction must either be underscore, appear in Expression or in VarSequence, or must appear before the comprehension expression in a query or rule and will have been given a ground value. This ensures that the value of a list comprehension is always a list of ground terms.

VarSequence is a single variable or a comma separated sequence of variables such as X,Y,Z

The syntax for SimpleConjunction is given below.