1proplists(3) Erlang Module Definition proplists(3)
2
6 proplists - Support functions for property lists.
7
9 Property lists are ordinary lists containing entries in the form of ei‐
10 ther tuples, whose first elements are keys used for lookup and inser‐
11 tion, or atoms, which work as shorthand for tuples {Atom, true}. (Other
12 terms are allowed in the lists, but are ignored by this module.) If
13 there is more than one entry in a list for a certain key, the first oc‐
14 currence normally overrides any later (irrespective of the arity of the
15 tuples).
16 Property lists are useful for representing inherited properties, such
18 as options passed to a function where a user can specify options over‐
19 riding the default settings, object properties, annotations, and so on.
20 Two keys are considered equal if they match (=:=). That is, numbers are
22 compared literally rather than by value, so that, for example, 1 and
23 1.0 are different keys.
24
26 property() = atom() | tuple()
27 proplist() = [property()]
29
31 append_values(Key, ListIn) -> ListOut
32 Types:
34 Key = term()
36 ListIn = ListOut = [term()]
37 Similar to get_all_values/2, but each value is wrapped in a list
39 unless it is already itself a list. The resulting list of lists
40 is concatenated. This is often useful for "incremental" options.
41 Example:
43 append_values(a, [{a, [1,2]}, {b, 0}, {a, 3}, {c, -1}, {a, [4]}])
45 returns:
47 [1,2,3,4]
49 compact(ListIn) -> ListOut
51 Types:
53 ListIn = ListOut = [property()]
55 Minimizes the representation of all entries in the list. This is
57 equivalent to [property(P) || P <- ListIn].
58 See also property/1, unfold/1.
60 delete(Key, List) -> List
62 Types:
64 Key = term()
66 List = [term()]
67 Deletes all entries associated with Key from List.
69 expand(Expansions, ListIn) -> ListOut
71 Types:
73 Expansions = [{Property :: property(), Expansion ::
75 [term()]}]
76 ListIn = ListOut = [term()]
77 Expands particular properties to corresponding sets of proper‐
79 ties (or other terms). For each pair {Property, Expansion} in
80 Expansions: if E is the first entry in ListIn with the same key
81 as Property, and E and Property have equivalent normal forms,
82 then E is replaced with the terms in Expansion, and any follow‐
83 ing entries with the same key are deleted from ListIn.
84 For example, the following expressions all return [fie, bar,
86 baz, fum]:
87 expand([{foo, [bar, baz]}], [fie, foo, fum])
89 expand([{{foo, true}, [bar, baz]}], [fie, foo, fum])
90 expand([{{foo, false}, [bar, baz]}], [fie, {foo, false}, fum])
91 However, no expansion is done in the following call because
93 {foo, false} shadows foo:
94 expand([{{foo, true}, [bar, baz]}], [{foo, false}, fie, foo, fum])
96 Notice that if the original property term is to be preserved in
98 the result when expanded, it must be included in the expansion
99 list. The inserted terms are not expanded recursively. If Expan‐
100 sions contains more than one property with the same key, only
101 the first occurrence is used.
102 See also normalize/2.
104 from_map(Map) -> List
106 Types:
108 Map = #{Key => Value}
110 List = [{Key, Value}]
111 Key = Value = term()
112 Converts the map Map to a property list.
114 get_all_values(Key, List) -> [term()]
116 Types:
118 Key = term()
120 List = [term()]
121 Similar to get_value/2, but returns the list of values for all
123 entries {Key, Value} in List. If no such entry exists, the re‐
124 sult is the empty list.
125 get_bool(Key, List) -> boolean()
127 Types:
129 Key = term()
131 List = [term()]
132 Returns the value of a boolean key/value option. If lookup(Key,
134 List) would yield {Key, true}, this function returns true, oth‐
135 erwise false.
136 See also get_value/2, lookup/2.
138 get_keys(List) -> [term()]
140 Types:
142 List = [term()]
144 Returns an unordered list of the keys used in List, not contain‐
146 ing duplicates.
147 get_value(Key, List) -> term()
149 Types:
151 Key = term()
153 List = [term()]
154 Equivalent to get_value(Key, List, undefined).
156 get_value(Key, List, Default) -> term()
158 Types:
160 Key = term()
162 List = [term()]
163 Default = term()
164 Returns the value of a simple key/value property in List. If
166 lookup(Key, List) would yield {Key, Value}, this function re‐
167 turns the corresponding Value, otherwise Default.
168 See also get_all_values/2, get_bool/2, get_value/2, lookup/2.
170 is_defined(Key, List) -> boolean()
172 Types:
174 Key = term()
176 List = [term()]
177 Returns true if List contains at least one entry associated with
179 Key, otherwise false.
180 lookup(Key, List) -> none | tuple()
182 Types:
184 Key = term()
186 List = [term()]
187 Returns the first entry associated with Key in List, if one ex‐
189 ists, otherwise returns none. For an atom A in the list, the tu‐
190 ple {A, true} is the entry associated with A.
191 See also get_bool/2, get_value/2, lookup_all/2.
193 lookup_all(Key, List) -> [tuple()]
195 Types:
197 Key = term()
199 List = [term()]
200 Returns the list of all entries associated with Key in List. If
202 no such entry exists, the result is the empty list.
203 See also lookup/2.
205 normalize(ListIn, Stages) -> ListOut
207 Types:
209 ListIn = [term()]
211 Stages = [Operation]
212 Operation =
213 {aliases, Aliases} |
214 {negations, Negations} |
215 {expand, Expansions}
216 Aliases = Negations = [{Key, Key}]
217 Expansions = [{Property :: property(), Expansion ::
218 [term()]}]
219 ListOut = [term()]
220 Passes ListIn through a sequence of substitution/expansion
222 stages. For an aliases operation, function substitute_aliases/2
223 is applied using the specified list of aliases:
224 * For a negations operation, substitute_negations/2 is applied
226 using the specified negation list.
227 * For an expand operation, function expand/2 is applied using
229 the specified list of expansions.
230 The final result is automatically compacted (compare compact/1).
232 Typically you want to substitute negations first, then aliases,
234 then perform one or more expansions (sometimes you want to pre-
235 expand particular entries before doing the main expansion). You
236 might want to substitute negations and/or aliases repeatedly, to
237 allow such forms in the right-hand side of aliases and expansion
238 lists.
239 See also substitute_negations/2.
241 property(PropertyIn) -> PropertyOut
243 Types:
245 PropertyIn = PropertyOut = property()
247 Creates a normal form (minimal) representation of a property. If
249 PropertyIn is {Key, true}, where Key is an atom, Key is re‐
250 turned, otherwise the whole term PropertyIn is returned.
251 See also property/2.
253 property(Key, Value) -> Property
255 Types:
257 Key = Value = term()
259 Property = atom() | {term(), term()}
260 Creates a normal form (minimal) representation of a simple
262 key/value property. Returns Key if Value is true and Key is an
263 atom, otherwise a tuple {Key, Value} is returned.
264 See also property/1.
266 split(List, Keys) -> {Lists, Rest}
268 Types:
270 List = Keys = [term()]
272 Lists = [[term()]]
273 Rest = [term()]
274 Partitions List into a list of sublists and a remainder. Lists
276 contains one sublist for each key in Keys, in the corresponding
277 order. The relative order of the elements in each sublist is
278 preserved from the original List. Rest contains the elements in
279 List that are not associated with any of the specified keys,
280 also with their original relative order preserved.
281 Example:
283 split([{c, 2}, {e, 1}, a, {c, 3, 4}, d, {b, 5}, b], [a, b, c])
285 returns:
287 {[[a], [{b, 5}, b],[{c, 2}, {c, 3, 4}]], [{e, 1}, d]}
289 substitute_aliases(Aliases, ListIn) -> ListOut
291 Types:
293 Aliases = [{Key, Key}]
295 Key = term()
296 ListIn = ListOut = [term()]
297 Substitutes keys of properties. For each entry in ListIn, if it
299 is associated with some key K1 such that {K1, K2} occurs in
300 Aliases, the key of the entry is changed to K2. If the same K1
301 occurs more than once in Aliases, only the first occurrence is
302 used.
303 For example, substitute_aliases([{color, colour}], L) replaces
305 all tuples {color, ...} in L with {colour, ...}, and all atoms
306 color with colour.
307 See also normalize/2, substitute_negations/2.
309 substitute_negations(Negations, ListIn) -> ListOut
311 Types:
313 Negations = [{Key1, Key2}]
315 Key1 = Key2 = term()
316 ListIn = ListOut = [term()]
317 Substitutes keys of boolean-valued properties and simultaneously
319 negates their values. For each entry in ListIn, if it is associ‐
320 ated with some key K1 such that {K1, K2} occurs in Negations: if
321 the entry was {K1, true}, it is replaced with {K2, false}, oth‐
322 erwise with K2, thus changing the name of the option and simul‐
323 taneously negating the value specified by get_bool(Key, ListIn).
324 If the same K1 occurs more than once in Negations, only the
325 first occurrence is used.
326 For example, substitute_negations([{no_foo, foo}], L) replaces
328 any atom no_foo or tuple {no_foo, true} in L with {foo, false},
329 and any other tuple {no_foo, ...} with foo.
330 See also get_bool/2, normalize/2, substitute_aliases/2.
332 to_map(List) -> Map
334 Types:
336 List = [Shorthand | {Key, Value} | term()]
338 Map = #{Shorthand => true, Key => Value}
339 Shorthand = atom()
340 Key = Value = term()
341 Converts the property list List to a map.
343 Shorthand atom values in List will be expanded to an association
345 of the form Atom => true. Tuples of the form {Key, Value} in
346 List will be converted to an association of the form Key =>
347 Value. Anything else will be silently ignored.
348 If the same key appears in List multiple times, the value of the
350 one appearing nearest to the head of List will be in the result
351 map, that is the value that would be returned by a call to
352 get_value(Key, List).
353 Example:
355 to_map([a, {b, 1}, {c, 2}, {c, 3}])
357 returns:
359 #{a => true, b => 1, c => 2}
361 to_map(List, Stages) -> Map
363 Types:
365 List = [term()]
367 Stages = [Operation]
368 Operation =
369 {aliases, Aliases} |
370 {negations, Negations} |
371 {expand, Expansions}
372 Aliases = Negations = [{Key, Key}]
373 Expansions = [{Property :: property(), Expansion ::
374 [term()]}]
375 Map = #{term() => term()}
376 Converts the property list List to a map after applying the nor‐
378 malizations given in Stages.
379 See also normalize/2, to_map/1.
381 unfold(ListIn) -> ListOut
383 Types:
385 ListIn = ListOut = [term()]
387 Unfolds all occurrences of atoms in ListIn to tuples {Atom,
389 true}.
390Ericsson AB stdlib 5.1.1 proplists(3)