/usr/local/lib/swipl/library/aggregate.pl
All Application Manual Name SummaryHelp

  • swipl
    • library
      • error.pl
      • debug.pl
      • apply.pl -- Apply predicates on a list
      • lists.pl
      • broadcast.pl -- Event service
      • predicate_options.pl
      • shlib.pl
      • option.pl
      • thread_pool.pl -- Resource bounded thread management
      • gensym.pl
      • settings.pl -- Setting management
      • arithmetic.pl -- Extensible arithmetic
      • main.pl
      • readutil.pl
      • operators.pl -- Manage operators
      • pairs.pl -- Operations on key-value lists
      • prolog_source.pl -- Examine Prolog source-files
      • record.pl -- Access compound arguments by name
      • quasi_quotations.pl -- Define Quasi Quotation syntax
      • pure_input.pl -- Pure Input from files and streams
      • solution_sequences.pl -- Modify solution sequences
      • ordsets.pl -- Ordered set manipulation
      • random.pl -- Random numbers
      • base64.pl -- Base64 encoding and decoding
      • aggregate.pl -- Aggregation operators on backtrackable predicates
        • aggregate/3
        • aggregate/4
        • aggregate_all/3
        • aggregate_all/4
        • foldall/4
        • foreach/2
        • free_variables/4
        • safe_meta/2
      • yall.pl -- Lambda expressions
      • sandbox.pl -- Sandboxed Prolog code
      • apply_macros.pl -- Goal expansion rules to avoid meta-calling
      • assoc.pl -- Binary associations
      • prolog_format.pl -- Analyse format specifications
      • csv.pl -- Process CSV (Comma-Separated Values) data
      • pprint.pl -- Pretty Print Prolog terms
      • atom.pl -- Operations on atoms
      • modules.pl -- Module utility predicates
      • occurs.pl -- Finding and counting sub-terms
      • prolog_xref.pl -- Prolog cross-referencer data collection
      • prolog_colour.pl
      • lazy_lists.pl -- Lazy list handling
      • ugraphs.pl -- Graph manipulation library
      • iostream.pl -- Utilities to deal with streams
      • url.pl -- Analysing and constructing URL
      • www_browser.pl -- Open a URL in the users browser
      • prolog_pack.pl -- A package manager for Prolog
      • git.pl -- Run GIT commands
      • utf8.pl -- UTF-8 encoding/decoding on lists of character codes.
      • dialect.pl -- Support multiple Prolog dialects
      • system.pl -- System utilities
      • terms.pl -- Term manipulation
      • date.pl -- Process dates and times
      • persistency.pl -- Provide persistent dynamic predicates
      • prolog_stack.pl -- Examine the Prolog stack
      • edinburgh.pl -- Some traditional Edinburgh predicates
      • prolog_clause.pl -- Get detailed source-information about a clause
      • prolog_breakpoints.pl -- Manage Prolog break-points
      • wfs.pl -- Well Founded Semantics interface
      • prolog_code.pl -- Utilities for reasoning about code
      • sort.pl
      • dicts.pl -- Dict utilities
      • dif.pl -- The dif/2 constraint
      • varnumbers.pl -- Utilities for numbered terms
      • pio.pl -- Pure I/O
      • backcomp.pl -- Backward compatibility
      • exceptions.pl -- Exception classification
      • prolog_jiti.pl -- Just In Time Indexing (JITI) utilities
      • ansi_term.pl -- Print decorated text to ANSI consoles
      • listing.pl -- List programs and pretty print clauses
      • charsio.pl -- I/O on Lists of Character Codes
      • threadutil.pl -- Interactive thread utilities
      • tables.pl -- XSB interface to tables
      • base32.pl -- Base32 encoding and decoding
      • codesio.pl -- I/O on Lists of Character Codes
      • coinduction.pl -- Co-Logic Programming
      • heaps.pl -- heaps/priority queues
      • rbtrees.pl -- Red black trees
      • statistics.pl -- Get information about resource usage
      • when.pl -- Conditional coroutining
      • qsave.pl -- Save current program as a state or executable
      • zip.pl -- Access resource ZIP archives
      • ctypes.pl -- Character code classification
      • strings.pl -- String utilities
      • prolog_versions.pl -- Demand specific (Prolog) versions
      • check.pl -- Consistency checking
      • optparse.pl -- command line parsing
      • prolog_profile.pl -- Execution profiler
      • hashtable.pl -- Hash tables
      • fastrw.pl -- Fast reading and writing of terms
      • quintus.pl -- Quintus compatibility
      • writef.pl -- Old-style formatted write
      • explain.pl -- Describe Prolog Terms
      • shell.pl -- Elementary shell commands
      • increval.pl -- Incremental dynamic predicate modification
      • thread.pl -- High level thread primitives
 foreach(:Generator, :Goal)
True when the conjunction of instances of Goal created from solutions for Generator is true. Except for term copying, this could be implemented as below.
foreach(Generator, Goal) :-
    findall(Goal, Generator, Goals),
    maplist(call, Goals).

The actual implementation uses findall/3 on a template created from the variables shared between Generator and Goal. Subsequently, it uses every instance of this template to instantiate Goal, call Goal and undo only the instantiation of the template and not other instantiations created by running Goal. Here is an example:

?- foreach(between(1,4,X), dif(X,Y)), Y = 5.
Y = 5.
?- foreach(between(1,4,X), dif(X,Y)), Y = 3.
false.

The predicate foreach/2 is mostly used if Goal performs backtrackable destructive assignment on terms. Attributed variables (underlying constraints) are an example. Another example of a backtrackable data structure is in library(hashtable). If we care only about the side effects (I/O, dynamic database, etc.) or the truth value of Goal, forall/2 is a faster and simpler alternative. If Goal instantiates its arguments it is will often fail as the argument cannot be instantiated to multiple values. It is possible to incrementally grow an argument:

?- foreach(between(1,4,X), member(X, L)).
L = [1,2,3,4|_].

Note that SWI-Prolog up to version 8.3.4 created copies of Goal using copy_term/2 for each iteration, this makes the current implementation unable to properly handle compound terms (in Goal's arguments) that share variables with the Generator. As a workaround you can define a goal that does not use compound terms, like in this example:

mem(E,L) :-  % mem/2 hides the compound argument from foreach/2
   member(r(E),L).

?- foreach(  between(1,5,N), mem(N,L)).