bagof/3 — The Logtalk Handbook v3.47.0 documentation

# `bagof/3`

## Description

    bagof(Template, Goal, List)

Collects a bag of solutions for the goal for each set of instantiations of the free variables in the goal. The order of the elements in the bag follows the order of the goal solutions. The free variables in the goal are the variables that occur in the goal but not in the template. Free variables can be ignored, however, by using the `^/2` existential qualifier. For example, if `T` is term containing all the free variables that we want to ignore, we can write `T^Goal`. Note that the term `T` can be written as `V1^V2^...`.

When there are free variables, this method is re-executable on backtracking. This method fails when there are no solutions, never returning an empty list.

This built-in meta-predicate is declared as a private method and thus cannot be used as a message to an object.

## Modes and number of proofs

    bagof(@term, +callable, -list) - zero_or_more

## Errors

`Goal` is a variable:
`instantiation_error`
`Goal` is neither a variable nor a callable term:
`type_error(callable, Goal)`
`Goal` is a call to a non-existing predicate:
`existence_error(procedure, Predicate)`

## Examples

To find a bag of solutions in the context of the object or category containing the call:
`bagof(Template, Goal, List)`
To find a bag of solutions of sending a message to self:
`bagof(Template, ::Message, List)`
To find a bag of solutions of sending a message to an explicit object:
`bagof(Template, Object::Message, List)`

See also

findall/3, findall/4, forall/2, setof/3