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
10 either 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
14 occurrence normally overrides any later (irrespective of the arity of
15 the 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 get_all_values(Key, List) -> [term()]
106 Types:
108 Key = term()
110 List = [term()]
111 Similar to get_value/2, but returns the list of values for all
113 entries {Key, Value} in List. If no such entry exists, the
114 result is the empty list.
115 get_bool(Key, List) -> boolean()
117 Types:
119 Key = term()
121 List = [term()]
122 Returns the value of a boolean key/value option. If lookup(Key,
124 List) would yield {Key, true}, this function returns true, oth‐
125 erwise false.
126 See also get_value/2, lookup/2.
128 get_keys(List) -> [term()]
130 Types:
132 List = [term()]
134 Returns an unordered list of the keys used in List, not contain‐
136 ing duplicates.
137 get_value(Key, List) -> term()
139 Types:
141 Key = term()
143 List = [term()]
144 Equivalent to get_value(Key, List, undefined).
146 get_value(Key, List, Default) -> term()
148 Types:
150 Key = term()
152 List = [term()]
153 Default = term()
154 Returns the value of a simple key/value property in List. If
156 lookup(Key, List) would yield {Key, Value}, this function
157 returns the corresponding Value, otherwise Default.
158 See also get_all_values/2, get_bool/2, get_value/2, lookup/2.
160 is_defined(Key, List) -> boolean()
162 Types:
164 Key = term()
166 List = [term()]
167 Returns true if List contains at least one entry associated with
169 Key, otherwise false.
170 lookup(Key, List) -> none | tuple()
172 Types:
174 Key = term()
176 List = [term()]
177 Returns the first entry associated with Key in List, if one
179 exists, otherwise returns none. For an atom A in the list, the
180 tuple {A, true} is the entry associated with A.
181 See also get_bool/2, get_value/2, lookup_all/2.
183 lookup_all(Key, List) -> [tuple()]
185 Types:
187 Key = term()
189 List = [term()]
190 Returns the list of all entries associated with Key in List. If
192 no such entry exists, the result is the empty list.
193 See also lookup/2.
195 normalize(ListIn, Stages) -> ListOut
197 Types:
199 ListIn = [term()]
201 Stages = [Operation]
202 Operation =
203 {aliases, Aliases} |
204 {negations, Negations} |
205 {expand, Expansions}
206 Aliases = Negations = [{Key, Key}]
207 Expansions = [{Property :: property(), Expansion ::
208 [term()]}]
209 ListOut = [term()]
210 Passes ListIn through a sequence of substitution/expansion
212 stages. For an aliases operation, function substitute_aliases/2
213 is applied using the specified list of aliases:
214 * For a negations operation, substitute_negations/2 is applied
216 using the specified negation list.
217 * For an expand operation, function expand/2 is applied using
219 the specified list of expansions.
220 The final result is automatically compacted (compare compact/1).
222 Typically you want to substitute negations first, then aliases,
224 then perform one or more expansions (sometimes you want to pre-
225 expand particular entries before doing the main expansion). You
226 might want to substitute negations and/or aliases repeatedly, to
227 allow such forms in the right-hand side of aliases and expansion
228 lists.
229 See also substitute_negations/2.
231 property(PropertyIn) -> PropertyOut
233 Types:
235 PropertyIn = PropertyOut = property()
237 Creates a normal form (minimal) representation of a property. If
239 PropertyIn is {Key, true}, where Key is an atom, Key is
240 returned, otherwise the whole term PropertyIn is returned.
241 See also property/2.
243 property(Key, Value) -> Property
245 Types:
247 Key = Value = term()
249 Property = atom() | {term(), term()}
250 Creates a normal form (minimal) representation of a simple
252 key/value property. Returns Key if Value is true and Key is an
253 atom, otherwise a tuple {Key, Value} is returned.
254 See also property/1.
256 split(List, Keys) -> {Lists, Rest}
258 Types:
260 List = Keys = [term()]
262 Lists = [[term()]]
263 Rest = [term()]
264 Partitions List into a list of sublists and a remainder. Lists
266 contains one sublist for each key in Keys, in the corresponding
267 order. The relative order of the elements in each sublist is
268 preserved from the original List. Rest contains the elements in
269 List that are not associated with any of the specified keys,
270 also with their original relative order preserved.
271 Example:
273 split([{c, 2}, {e, 1}, a, {c, 3, 4}, d, {b, 5}, b], [a, b, c])
275 returns:
277 {[[a], [{b, 5}, b],[{c, 2}, {c, 3, 4}]], [{e, 1}, d]}
279 substitute_aliases(Aliases, ListIn) -> ListOut
281 Types:
283 Aliases = [{Key, Key}]
285 Key = term()
286 ListIn = ListOut = [term()]
287 Substitutes keys of properties. For each entry in ListIn, if it
289 is associated with some key K1 such that {K1, K2} occurs in
290 Aliases, the key of the entry is changed to K2. If the same K1
291 occurs more than once in Aliases, only the first occurrence is
292 used.
293 For example, substitute_aliases([{color, colour}], L) replaces
295 all tuples {color, ...} in L with {colour, ...}, and all atoms
296 color with colour.
297 See also normalize/2, substitute_negations/2.
299 substitute_negations(Negations, ListIn) -> ListOut
301 Types:
303 Negations = [{Key1, Key2}]
305 Key1 = Key2 = term()
306 ListIn = ListOut = [term()]
307 Substitutes keys of boolean-valued properties and simultaneously
309 negates their values. For each entry in ListIn, if it is associ‐
310 ated with some key K1 such that {K1, K2} occurs in Negations: if
311 the entry was {K1, true}, it is replaced with {K2, false}, oth‐
312 erwise with K2, thus changing the name of the option and simul‐
313 taneously negating the value specified by get_bool(Key, ListIn).
314 If the same K1 occurs more than once in Negations, only the
315 first occurrence is used.
316 For example, substitute_negations([{no_foo, foo}], L) replaces
318 any atom no_foo or tuple {no_foo, true} in L with {foo, false},
319 and any other tuple {no_foo, ...} with foo.
320 See also get_bool/2, normalize/2, substitute_aliases/2.
322 unfold(ListIn) -> ListOut
324 Types:
326 ListIn = ListOut = [term()]
328 Unfolds all occurrences of atoms in ListIn to tuples {Atom,
330 true}.
331Ericsson AB stdlib 3.14.1 proplists(3)