SWI-Prolog version 7 introduces dicts as an abstract object with
a concrete modern syntax and functional notation for accessing members
and as well as access functions defined by the user. The syntax for a
dict is illustrated below. Tag is either a variable or an
atom. As with compound terms, there is no space between the tag
and the opening brace. The keys are either atoms or small integers (up
to
max_tagged_integer).
The values are arbitrary Prolog terms which are parsed using the same
rules as used for arguments in compound terms.
Tag{Key1:Value1, Key2:Value2, ...}
A dict can not hold duplicate keys. The dict is transformed
into an opaque internal representation that does not respect
the order in which the key-value pairs appear in the input text. If a
dict is written, the keys are written according to the standard order of
terms (see section 4.6.1).
Here are some examples, where the second example illustrates that the
order is not maintained and the third illustrates an anonymous dict.
?- A = point{x:1, y:2}.
A = point{x:1, y:2}.
?- A = point{y:2, x:1}.
A = point{x:1, y:2}.
?- A = _{first_name:"Mel", last_name:"Smith"}.
A = _G1476{first_name:"Mel", last_name:"Smith"}.
Dicts can be unified following the standard symmetric Prolog
unification rules. As dicts use an internal canonical form, the order in
which the named keys are represented is not relevant. This behaviour is
illustrated by the following example.
?- point{x:1, y:2} = Tag{y:2, x:X}.
Tag = point,
X = 1.
Note In the current implementation, two dicts unify only if
they have the same set of keys and the tags and values associated with
the keys unify. In future versions, the notion of unification between
dicts could be modified such that two dicts unify if their tags and the
values associated with common keys unify, turning both dicts
into a new dict that has the union of the keys of the two original
dicts.
The infix operator dot (op(100, yfx, .) is used to
extract values and evaluate functions on dicts. Functions are recognised
if they appear in the argument of a goal in the source text,
possibly nested in a term. The keys act as field selector, which is
illustrated in this example.
?- X = point{x:1,y:2}.x.
X = 1.
?- Pt = point{x:1,y:2}, write(Pt.y).
2
Pt = point{x:1,y:2}.
?- X = point{x:1,y:2}.C.
X = 1,
C = x ;
X = 2,
C = y.
The compiler translates a goal that contains ./2system module. Terms functor.2 that appears in
the head are replaced with a variable and calls to ./3
are inserted at the start of the body. Below are two examples, where the
first extracts the
x key from a dict and the second extends a dict containing
an address with the postal code, given a find_postal_code/4 predicate.
dict_x(X, X.x).
add_postal_code(Dict, Dict.put(postal_code, Code)) :-
find_postal_code(Dict.city,
Dict.street,
Dict.house_number,
Code).
Note that expansion of ./2./2./2
- .(+Dict, +Function,
-Result)
- This predicate is called to evaluate
./2
The tag of a dict associates the dict to a module. If the dot
notation uses a compound term, this calls the goal below.
<module>:<name>(Arg1, ..., +Dict,
-Value)
Functions are normal Prolog predicates. The dict infrastructure
provides a more convenient syntax for representing the head of such
predicates without worrying about the argument calling conventions. The
code below defines a function multiply(Times) on a point
that creates a new point by multiplying both coordinates. and len182as length
would result in a predicate length/2,
this name cannot be used. This might change in future versions.
to compute the length from the origin. The . and :=
operators are used to abstract the location of the predicate arguments.
It is allowed to define multiple a function with multiple clauses,
providing overloading and non-determinism.
:- module(point, []).
M.multiply(F) := point{x:X, y:Y} :-
X is M.x*F,
Y is M.y*F.
M.len() := Len :-
Len is sqrt(M.x**2 + M.y**2).
After these definitions, we can evaluate the following functions:
?- X = point{x:1, y:2}.multiply(2).
X = point{x:2, y:4}.
?- X = point{x:1, y:2}.multiply(2).len().
X = 4.47213595499958.
Dicts currently define the following reserved functions:
- get(?KeyPath)
- Return the value associates with KeyPath. KeyPath
is either a single key or a term
Key1/Key2/.... Each key is
either an atom, small integer or a variable. While Dict.Key
throws an existence error, this function fails silently if a
key does not exist in the target dict. See also :</2,
which can be used to test for existence and unify multiple key values
from a dict. For example:
?- write(t{a:x}.get(a)).
x
?- write(t{a:x}.get(b)).
false.
?- write(t{a:t{b:x}}.get(a/b)).
x
- put(+New)
- Evaluates to a new dict where the key-values in New replace
or extend the key-values in the original dict. See put_dict/3.
- get(?KeyPath,
+Default)
- Same as get/1 , but if no match is found the function evaluates to Default.
If KeyPath contains variables possible choice points are
respected and the function only evaluates to Default if the
pattern has no matches.
- put(+KeyPath,
+Value)
- Evaluates to a new dict where the KeyPath-Value
replaces or extends the key-values in the original dict. KeyPath
is either a key or a term KeyPath/Key,183Note
that we do not use the’.’functor here, because the
./2
?- A = _{}.put(a, 1).
A = _G7359{a:1}.
?- A = _{a:1}.put(a, 2).
A = _G7377{a:2}.
?- A = _{a:1}.put(b/c, 2).
A = _G1395{a:1, b:_G1584{c:2}}.
?- A = _{a:_{b:1}}.put(a/b, 2).
A = _G1429{a:_G1425{b:2}}.
?- A = _{a:1}.put(a/b, 2).
A = _G1395{a:_G1578{b:2}}.
This section documents the predicates that are defined on dicts. We
use the naming and argument conventions of the traditional library(assoc).
- is_dict(@Term)
- True if Term is a dict. This is the same as
is_dict(Term,_).
- is_dict(@Term,
-Tag)
- True if Term is a dict of Tag.
- get_dict(?Key,
+Dict, -Value)
- Unify the value associated with Key in dict with Value.
If
Key is unbound, all associations in Dict are
returned on backtracking. The order in which the associations are
returned is undefined. This predicate is normally accessed using the
functional notation
Dict.Key. See section
5.4.1.
Fails silently if Key does not appear in Dict. This is different from
the behavior of the functional‘.`-notation, which throws an
existence error in that case.
- [semidet]get_dict(+Key,
+Dict, -Value, -NewDict, +NewValue)
- Create a new dict after updating the value for Key. Fails if
Value does not unify with the current value associated with
Key. Dict is either a dict or a list the can be
converted into a dict.
Has the behavior as if defined in the following way:
get_dict(Key, Dict, Value, NewDict, NewValue) :-
get_dict(Key, Dict, Value),
put_dict(Key, Dict, NewValue, NewDict).
- dict_create(-Dict,
+Tag, +Data)
- Create a dict in Tag from Data. Data is
a list of attribute-value pairs using the syntax
Key:Value,
Key=Value, Key-Value or Key(Value).
An exception is raised if Data is not a proper list, one of
the elements is not of the shape above, a key is neither an atom nor a
small integer or there is a duplicate key.
- dict_pairs(?Dict,
?Tag, ?Pairs)
- Bi-directional mapping between a dict and an ordered list of pairs (see section
A.34).
- dict_same_keys(?Dict1,
?Dict2)
- True when Dict1 and Dict2 have the same set of
keys. The
tag is not considered. This predicate is semidet if both
arguments are instantiated to a dict. If one is instantiated to a dict
and the other is unbound, it generates a dict with the same keys and
unbound tag and values. The predicate is_dict/2
may be used test tag equivalence or unify the tags. This predicate
raises an
instantiation_error if both argument are unbound
or a type_error if one of the arguments is neither a dict
nor a variable.
- put_dict(+New,
+DictIn, -DictOut)
- DictOut is a new dict created by replacing or adding
key-value pairs from New to Dict. New
is either a dict or a valid input for dict_create/3.
This predicate is normally accessed using the functional notation. Below
are some examples:
?- A = point{x:1, y:2}.put(_{x:3}).
A = point{x:3, y:2}.
?- A = point{x:1, y:2}.put([x=3]).
A = point{x:3, y:2}.
?- A = point{x:1, y:2}.put([x=3,z=0]).
A = point{x:3, y:2, z:0}.
- put_dict(+Key,
+DictIn, +Value, -DictOut)
- DictOut is a new dict created by replacing or adding
Key-Value to DictIn. For example:
?- A = point{x:1, y:2}.put(x, 3).
A = point{x:3, y:2}.
This predicate can also be accessed by using the functional notation,
in which case Key can also be a *path* of keys. For example:
?- Dict = _{}.put(a/b, c).
Dict = _6096{a:_6200{b:c}}.
- del_dict(+Key,
+DictIn, ?Value, -DictOut)
- True when Key-Value is in DictIn and DictOut
contains all associations of DictIn except for Key.
- [semidet]+Select :< +From
- True when Select is a‘sub dict’of From:
the tags must unify and all keys in Select must appear with
unifying values in From. From may contain keys
that are not in
Select. This operation is frequently used to match a
dict and at the same time extract relevant values from it. For example:
plot(Dict, On) :-
_{x:X, y:Y, z:Z} :< Dict, !,
plot_xyz(X, Y, Z, On).
plot(Dict, On) :-
_{x:X, y:Y} :< Dict, !,
plot_xy(X, Y, On).
The goal Select :< From is equivalent to
select_dict(Select, From, _).
- [semidet]select_dict(+Select,
+From, -Rest)
- True when the tags of Select and From have been
unified, all keys in Select appear in From and the
corresponding values have been unified. The key-value pairs of From
that do not appear in Select are used to form an anonymous
dict, which is unified with Rest. For example:
?- select_dict(P{x:0, y:Y}, point{x:0, y:1, z:2}, R).
P = point,
Y = 1,
R = _{z:2}.
See also :</2 to
ignore Rest and >:</2
for a symmetric partial unification of two dicts.
- +Dict1 >:< +Dict2
- This operator specifies a partial unification between
Dict1 and Dict2. It is true when the tags and the
values associated with all common keys have been unified. The
values associated to keys that do not appear in the other dict are
ignored. Partial unification is symmetric. For example, given a list of
dicts, find dicts that represent a point with X equal to zero:
member(Dict, List),
Dict >:< point{x:0, y:Y}.
See also :</2 and select_dict/3.
This section describes the destructive update operations defined on
dicts. These actions can only update keys and not add or remove
keys. If the requested key does not exist the predicate raises
existence_error(key, Key, Dict). Note the additional
argument.
Destructive assignment is a non-logical operation and should be used
with care because the system may copy or share identical Prolog terms at
any time. Some of this behaviour can be avoided by adding an additional
unbound value to the dict. This prevents unwanted sharing and ensures
that copy_term/2
actually copies the dict. This pitfall is demonstrated in the example
below:
?- A = a{a:1}, copy_term(A,B), b_set_dict(a, A, 2).
A = B, B = a{a:2}.
?- A = a{a:1,dummy:_}, copy_term(A,B), b_set_dict(a, A, 2).
A = a{a:2, dummy:_G3195},
B = a{a:1, dummy:_G3391}.
- [det]b_set_dict(+Key,
!Dict, +Value)
- Destructively update the value associated with Key in Dict
to
Value. The update is trailed and undone on backtracking. This
predicate raises an existence error if Key does not appear in
Dict. The update semantics are equivalent to setarg/3
and
b_setval/2.
- [det]nb_set_dict(+Key,
!Dict, +Value)
- Destructively update the value associated with Key in Dict
to a copy of Value. The update is not undone on
backtracking. This predicate raises an existence error if Key
does not appear in
Dict. The update semantics are equivalent to nb_setarg/3
and
nb_setval/2.
- [det]nb_link_dict(+Key,
!Dict, +Value)
- Destructively update the value associated with Key in Dict
to
Value. The update is not undone on backtracking.
This predicate raises an existence error if Key does not
appear in
Dict. The update semantics are equivalent to nb_linkarg/3
and
nb_linkval/2.
Use with extreme care and consult the documentation of
nb_linkval/2
before use.
Dicts are a new type in the Prolog world. They compete with several
other types and libraries. In the list below we have a closer look at
these relations. We will see that dicts are first of all a good
replacement for compound terms with a high or not clearly fixed arity,
library
library(record) and option processing.
- Compound terms
- Compound terms with positional arguments form the traditional way to
package data in Prolog. This representation is well understood, fast and
compound terms are stored efficiently. Compound terms are still the
representation of choice, provided that the number of arguments is low
and fixed or compactness or performance are of utmost importance.
A good example of a compound term is the representation of RDF
triples using the term rdf(Subject, Predicate, Object)
because RDF triples are defined to have precisely these three arguments
and they are always referred to in this order. An application processing
information about persons should probably use dicts because the
information that is related to a person is not so fixed. Typically we
see first and last name. But there may also be title, middle name,
gender, date of birth, etc. The number of arguments becomes unmanageable
when using a compound term, while adding or removing an argument leads
to many changes in the program.
- Library
library(record) - Using library
library(record) relieves the maintenance
issues associated with using compound terms significantly. The library
generates access and modification predicates for each field in a
compound term from a declaration. The library provides sound access to
compound terms with many arguments. One of its problems is the verbose
syntax needed to access or modify fields which results from long names
for the generated predicates and the restriction that each field needs
to be extracted with a separate goal. Consider the example below, where
the first uses library library(record) and the second uses
dicts.
...,
person_first_name(P, FirstName),
person_last_name(P, LastName),
format('Dear ~w ~w,~n~n', [FirstName, LastName]).
...,
format('Dear ~w ~w,~n~n', [Dict.first_name, Dict.last_name]).
Records have a fixed number of arguments and (non-)existence of an
argument must be represented using a value that is outside the normal
domain. This lead to unnatural code. For example, suppose our person
also has a title. If we know the first name we use this and else we use
the title. The code samples below illustrate this.
salutation(P) :-
person_first_name(P, FirstName), nonvar(FirstName), !,
person_last_name(P, LastName),
format('Dear ~w ~w,~n~n', [FirstName, LastName]).
salutation(P) :-
person_title(P, Title), nonvar(Title), !,
person_last_name(P, LastName),
format('Dear ~w ~w,~n~n', [Title, LastName]).
salutation(P) :-
_{first_name:FirstName, last_name:LastName} :< P, !,
format('Dear ~w ~w,~n~n', [FirstName, LastName]).
salutation(P) :-
_{title:Title, last_name:LastName} :< P, !,
format('Dear ~w ~w,~n~n', [Title, LastName]).
- Library
library(assoc) - This library implements a balanced binary tree. Dicts can replace the
use of this library if the association is fairly static (i.e., there are
few update operations), all keys are atoms or (small) integers and the
code does not rely on ordered operations.
- Library
library(option) - Option lists are introduced by ISO Prolog, for example for read_term/3,
open/4,
etc. The
library(option) library provides operations to
extract options, merge options lists, etc. Dicts are well suited to
replace option lists because they are cheaper, can be processed faster
and have a more natural syntax.
- Library
library(pairs) - This library is commonly used to process large name-value associations.
In many cases this concerns short-lived data structures that result from
findall/3, maplist/3
and similar list processing predicates. Dicts may play a role if
frequent random key lookups are needed on the resulting association. For
example, the skeleton‘create a pairs list’,‘use
list_to_assoc/2
to create an assoc’, followed by frequent usage of
get_assoc/3
to extract key values can be replaced using dict_pairs/3
and the dict access functions. Using dicts in this scenario is more
efficient and provides a more pleasant access syntax.
Dicts, or key-value associations, are a common data structure. A good
old example are property lists as found in Lisp, while a good
recent example is formed by JavaScript objects. Traditional
Prolog does not offer native property lists. As a result, people are
using a wide range of data structures for key-value associations:
- Using compound terms and positional arguments, e.g.,
point(1,2).
- Using compound terms with library
library(record),
which generates access predicates for a term using positional arguments
from a description.
- Using lists of terms
Name=Value, Name-Value,
Name:Value or Name(Value).
- Using library
library(assoc) which represents the
associations as a balanced binary tree.
This situation is unfortunate. Each of these have their advantages
and disadvantages. E.g., compound terms are compact and fast, but
inflexible and using positional arguments quickly breaks down. Library
library(record) fixes this, but the syntax is considered
hard to use. Lists are flexible, but expensive and the alternative
key-value representations that are used complicate the matter even more.
Library
library(assoc) allows for efficient manipulation of
changing associations, but the syntactical representation of an assoc is
complex, which makes them unsuitable for e.g., options lists as
seen in predicates such as open/4.
Although dicts are designed as an abstract data type and we
deliberately reserve the possibility to change the representation and
even use multiple representations, this section describes the current
implementation.
Dicts are currently represented as a compound term using the functor
`dict`. The first argument is the tag. The remaining
arguments create an array of sorted key-value pairs. This representation
is compact and guarantees good locality. Lookup is order log( N ),
while adding values, deleting values and merging with other dicts has
order
N. The main disadvantage is that changing values in large
dicts is costly, both in terms of memory and time.
Future versions may share keys in a separate structure or use a
binary trees to allow for cheaper updates. One of the issues is that the
representation must either be kept canonical or unification must be
extended to compensate for alternate representations.
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