1cerl_clauses(3) Erlang Module Definition cerl_clauses(3)
2
6 cerl_clauses - Utility functions for Core Erlang case/receive clauses.
7
9 Utility functions for Core Erlang case/receive clauses.
10 Syntax trees are defined in the module cerl.
12
14 bindings() = [{cerl:cerl(), cerl:cerl()}]:
15 cerl() = cerl:cerl():
18 expr() = any | cerl:cerl():
21 match_ret() = none | {true, bindings()} | {false, bindings()}:
24
27 any_catchall(Cs::[cerl:cerl()]) -> boolean()
28 Returns true if any of the abstract clauses in the list is a
30 catch-all, otherwise false. See is_catchall/1 for details.
31 Note: each node in Clauses must have type clause.
33 See also: is_catchall/1.
35 eval_guard(E::cerl:cerl()) -> none | {value, term()}
37 Tries to reduce a guard expression to a single constant value,
39 if possible. The returned value is {value, Term} if the guard
40 expression Expr always yields the constant value Term, and is
41 otherwise none.
42 Note that although guard expressions should only yield boolean
44 values, this function does not guarantee that Term is either
45 true or false. Also note that only simple constructs like let-
46 expressions are examined recursively; general constant folding
47 is not performed.
48 See also: is_catchall/1.
50 is_catchall(C::cerl:c_clause()) -> boolean()
52 Returns true if an abstract clause is a catch-all, otherwise
54 false. A clause is a catch-all if all its patterns are vari‐
55 ables, and its guard expression always evaluates to true; cf.
56 eval_guard/1.
57 Note: Clause must have type clause.
59 See also: any_catchall/1, eval_guard/1.
61 match(P::cerl:cerl(), E::expr()) -> match_ret()
63 Matches a pattern against an expression. The returned value is
65 none if a match is impossible, {true, Bindings} if Pattern defi‐
66 nitely matches Expr, and {false, Bindings} if a match is not
67 definite, but cannot be excluded. Bindings is then a list of
68 pairs {Var, SubExpr}, associating each variable in the pattern
69 with either the corresponding subexpression of Expr, or with the
70 atom any if no matching subexpression exists. (Recall that vari‐
71 ables may not be repeated in a Core Erlang pattern.) The list of
72 bindings is given in innermost-first order; this should only be
73 of interest if Pattern contains one or more alias patterns. If
74 the returned value is {true, []}, it implies that the pattern
75 and the expression are syntactically identical.
76 Instead of a syntax tree, the atom any can be passed for Expr
78 (or, more generally, be used for any subtree of Expr, in as much
79 the abstract syntax tree implementation allows it); this means
80 that it cannot be decided whether the pattern will match or not,
81 and the corresponding variable bindings will all map to any. The
82 typical use is for producing bindings for receive clauses.
83 Note: Binary-syntax patterns are never structurally matched
85 against binary-syntax expressions by this function.
86 Examples:
88 * Matching a pattern "{X, Y}" against the expression "{foo,
90 f(Z)}" yields {true, Bindings} where Bindings associates "X"
91 with the subtree "foo" and "Y" with the subtree "f(Z)".
92 * Matching pattern "{X, {bar, Y}}" against expression "{foo,
94 f(Z)}" yields {false, Bindings} where Bindings associates
95 "X" with the subtree "foo" and "Y" with any (because it is
96 not known if "{foo, Y}" might match the run-time value of
97 "f(Z)" or not).
98 * Matching pattern "{foo, bar}" against expression "{foo,
100 f()}" yields {false, []}, telling us that there might be a
101 match, but we cannot deduce any bindings.
102 * Matching {foo, X = {bar, Y}} against expression "{foo, {bar,
104 baz}}" yields {true, Bindings} where Bindings associates "Y"
105 with "baz", and "X" with "{bar, baz}".
106 * Matching a pattern "{X, Y}" against any yields {false, Bind‐
108 ings} where Bindings associates both "X" and "Y" with any.
109 match_list(Ps::[cerl:cerl()], Es::[expr()]) -> match_ret()
111 Like match/2, but matching a sequence of patterns against a se‐
113 quence of expressions. Passing an empty list for Exprs is equiv‐
114 alent to passing a list of any atoms of the same length as Pat‐
115 terns.
116 See also: match/2.
118 reduce(Cs::[cerl:c_clause()]) -> {true, {cerl:c_clause(), bindings()}}
120 | {false, [cerl:c_clause()]}
121 Equivalent to reduce(Cs, []).
123 reduce(Cs::[cerl:c_clause()], Es::[expr()]) -> {true, {cerl:c_clause(),
125 bindings()}} | {false, [cerl:c_clause()]}
126 Selects a single clause, if possible, or otherwise reduces the
128 list of selectable clauses. The input is a list Clauses of ab‐
129 stract clauses (i.e., syntax trees of type clause), and a list
130 of switch expressions Exprs. The function tries to uniquely se‐
131 lect a single clause or discard unselectable clauses, with re‐
132 spect to the switch expressions. All abstract clauses in the
133 list must have the same number of patterns. If Exprs is not the
134 empty list, it must have the same length as the number of pat‐
135 terns in each clause; see match_list/2 for details.
136 A clause can only be selected if its guard expression always
138 yields the atom true, and a clause whose guard expression always
139 yields the atom false can never be selected. Other guard expres‐
140 sions are considered to have unknown value; cf. eval_guard/1.
141 If a particular clause can be selected, the function returns
143 {true, {Clause, Bindings}}, where Clause is the selected clause
144 and Bindings is a list of pairs {Var, SubExpr} associating the
145 variables occurring in the patterns of Clause with the corre‐
146 sponding subexpressions in Exprs. The list of bindings is given
147 in innermost-first order; see the match/2 function for details.
148 If no clause could be definitely selected, the function returns
150 {false, NewClauses}, where NewClauses is the list of entries in
151 Clauses that remain after eliminating unselectable clauses, pre‐
152 serving the relative order.
153 See also: eval_guard/1, match/2, match_list/2.
155
157 Richard Carlsson <carlsson.richard@gmail.com>
158 compiler 8.2.6.3 cerl_clauses(3)