1merl(3) Erlang Module Definition merl(3)
2
6 merl - Metaprogramming in Erlang.
7
9 Metaprogramming in Erlang. Merl is a more user friendly interface to
10 the erl_syntax module, making it easy both to build new ASTs from
11 scratch and to match and decompose existing ASTs. For details that are
12 outside the scope of Merl itself, please see the documentation of
13 erl_syntax.
14 Quick start
16 To enable the full power of Merl, your module needs to include the Merl
18 header file:
19 -include_lib("syntax_tools/include/merl.hrl").
21 Then, you can use the ?Q(Text) macros in your code to create ASTs or
23 match on existing ASTs. For example:
24 Tuple = ?Q("{foo, 42}"),
26 ?Q("{foo, _@Number}") = Tuple,
27 Call = ?Q("foo:bar(_@Number)")
28 Calling merl:print(Call) will then print the following code:
30 foo:bar(42)
32 The ?Q macros turn the quoted code fragments into ASTs, and lifts
34 metavariables such as _@Tuple and _@Number to the level of your Erlang
35 code, so you can use the corresponding Erlang variables Tuple and Num‐
36 ber directly. This is the most straightforward way to use Merl, and in
37 many cases it's all you need.
38 You can even write case switches using ?Q macros as patterns. For exam‐
40 ple:
41 case AST of
43 ?Q("{foo, _@Foo}") -> handle(Foo);
44 ?Q("{bar, _@Bar}") when erl_syntax:is_integer(Bar) -> handle(Bar);
45 _ -> handle_default()
46 end
47 These case switches only allow ?Q(...) or _ as clause patterns, and the
49 guards may contain any expressions, not just Erlang guard expressions.
50 If the macro MERL_NO_TRANSFORM is defined before the merl.hrl header
52 file is included, the parse transform used by Merl will be disabled,
53 and in that case, the match expressions ?Q(...) = ..., case switches
54 using ?Q(...) patterns, and automatic metavariables like _@Tuple cannot
55 be used in your code, but the Merl macros and functions still work. To
56 do metavariable substitution, you need to use the ?Q(Text, Map) macro,
57 e.g.:
58 Tuple = ?Q("{foo, _@bar, _@baz}", [{bar, Bar}, {baz,Baz}])
60 The text given to a ?Q(Text) macro can be either a single string, or a
62 list of strings. The latter is useful when you need to split a long ex‐
63 pression over multiple lines, e.g.:
64 ?Q(["case _@Expr of",
66 " {foo, X} -> f(X);",
67 " {bar, X} -> g(X)",
68 " _ -> h(X)"
69 "end"])
70 If there is a syntax error somewhere in the text (like the missing
72 semicolon in the second clause above) this allows Merl to generate an
73 error message pointing to the exact line in your source code. (Just re‐
74 member to comma-separate the strings in the list, otherwise Erlang will
75 concatenate the string fragments as if they were a single string.)
76 Metavariable syntax
78 There are several ways to write a metavariable in your quoted code:
80 * Atoms starting with @, for example '@foo' or '@Foo'
82 * Variables starting with _@, for example _@bar or _@Bar
84 * Strings starting with "'@, for example "'@File"
86 * Integers starting with 909, for example 9091 or 909123
88 Following the prefix, one or more _ or 0 characters may be used to in‐
90 dicate "lifting" of the variable one or more levels, and after that, a
91 @ or 9 character indicates a glob metavariable (matching zero or more
92 elements in a sequence) rather than a normal metavariable. For example:
93 * '@_foo' is lifted one level, and _@__foo is lifted two levels
95 * _@@bar is a glob variable, and _@_@bar is a lifted glob variable
97 * 90901 is a lifted variable,90991 is a glob variable, and 9090091 is
99 a glob variable lifted two levels
100 (Note that the last character in the name is never considered to be a
102 lift or glob marker, hence, _@__ and 90900 are only lifted one level,
103 not two. Also note that globs only matter for matching; when doing sub‐
104 stitutions, a non-glob variable can be used to inject a sequence of el‐
105 ements, and vice versa.)
106 If the name after the prefix and any lift and glob markers is _ or 0,
108 the variable is treated as an anonymous catch-all pattern in matches.
109 For example, _@_, _@@_, _@__, or even _@__@_.
110 Finally, if the name without any prefixes or lift/glob markers begins
112 with an uppercase character, as in _@Foo or _@_@Foo, it will become a
113 variable on the Erlang level, and can be used to easily deconstruct and
114 construct syntax trees:
115 case Input of
117 ?Q("{foo, _@Number}") -> ?Q("foo:bar(_@Number)");
118 ...
119 We refer to these as "automatic metavariables". If in addition the name
121 ends with @, as in _@Foo@, the value of the variable as an Erlang term
122 will be automatically converted to the corresponding abstract syntax
123 tree when used to construct a larger tree. For example, in:
124 Bar = {bar, 42},
126 Foo = ?Q("{foo, _@Bar@}")
127 (where Bar is just some term, not a syntax tree) the result Foo will be
129 a syntax tree representing {foo, {bar, 42}}. This avoids the need for
130 temporary variables in order to inject data, as in
131 TmpBar = erl_syntax:abstract(Bar),
133 Foo = ?Q("{foo, _@TmpBar}")
134 If the context requires an integer rather than a variable, an atom, or
136 a string, you cannot use the uppercase convention to mark an automatic
137 metavariable. Instead, if the integer (without the 909-prefix and
138 lift/glob markers) ends in a 9, the integer will become an Erlang-level
139 variable prefixed with Q, and if it ends with 99 it will also be auto‐
140 matically abstracted. For example, the following will increment the ar‐
141 ity of the exported function f:
142 case Form of
144 ?Q("-export([f/90919]).") ->
145 Q2 = erl_syntax:concrete(Q1) + 1,
146 ?Q("-export([f/909299]).");
147 ...
148 When to use the various forms of metavariables
150 Merl can only parse a fragment of text if it follows the basic syntac‐
152 tical rules of Erlang. In most places, a normal Erlang variable can be
153 used as metavariable, for example:
154 ?Q("f(_@Arg)") = Expr
156 but if you want to match on something like the name of a function, you
158 have to use an atom as metavariable:
159 ?Q("'@Name'() -> _@@_." = Function
161 (note the anonymous glob variable _@@_ to ignore the function body).
163 In some contexts, only a string or an integer is allowed. For example,
165 the directive -file(Name, Line) requires that Name is a string literal
166 and Line an integer literal:
167 ?Q("-file(\"'@File\", 9090).") = ?Q("-file(\"foo.erl\", 42).")).
169 This will extract the string literal "foo.erl" into the variable Foo.
171 Note the use of the anonymous variable 9090 to ignore the line number.
172 To match and also bind a metavariable that must be an integer literal,
173 we can use the convention of ending the integer with a 9, turning it
174 into a Q-prefixed variable on the Erlang level (see the previous sec‐
175 tion).
176 Globs
178 Whenever you want to match out a number of elements in a sequence (zero
180 or more) rather than a fixed set of elements, you need to use a glob.
181 For example:
182 ?Q("{_@@Elements}") = ?Q({a, b, c})
184 will bind Elements to the list of individual syntax trees representing
186 the atoms a, b, and c. This can also be used with static prefix and
187 suffix elements in the sequence. For example:
188 ?Q("{a, b, _@@Elements}") = ?Q({a, b, c, d})
190 will bind Elements to the list of the c and d subtrees, and
192 ?Q("{_@@Elements, c, d}") = ?Q({a, b, c, d})
194 will bind Elements to the list of the a and b subtrees. You can even
196 use plain metavariables in the prefix or suffix:
197 ?Q("{_@First, _@@Rest}") = ?Q({a, b, c})
199 or
201 ?Q("{_@@_, _@Last}") = ?Q({a, b, c})
203 (ignoring all but the last element). You cannot however have two globs
205 as part of the same sequence.
206 Lifted metavariables
208 In some cases, the Erlang syntax rules make it impossible to place a
210 metavariable directly where you would like it. For example, you cannot
211 write:
212 ?Q("-export([_@@Name]).")
214 to match out all name/arity pairs in the export list, or to insert a
216 list of exports in a declaration, because the Erlang parser only allows
217 elements on the form A/I (where A is an atom and I an integer) in the
218 export list. A variable like the above is not allowed, but neither is a
219 single atom or integer, so '@@Name' or 909919 wouldn't work either.
220 What you have to do in such cases is to write your metavariable in a
222 syntactically valid position, and use lifting markers to denote where
223 it should really apply, as in:
224 ?Q("-export(['@_@Name'/0]).")
226 This causes the variable to be lifted (after parsing) to the next
228 higher level in the syntax tree, replacing that entire subtree. In this
229 case, the '@_@Name'/0 will be replaced with '@@Name', and the /0 part
230 was just used as dummy notation and will be discarded.
231 You may even need to apply lifting more than once. To match the entire
233 export list as a single syntax tree, you can write:
234 ?Q("-export(['@__Name'/0]).")
236 using two underscores, but with no glob marker this time. This will
238 make the entire ['@__Name'/0] part be replaced with '@Name'.
239 Sometimes, the tree structure of a code fragment isn't very obvious,
241 and parts of the structure may be invisible when printed as source
242 code. For instance, a simple function definition like the following:
243 zero() -> 0.
245 consists of the name (the atom zero), and a list of clauses containing
247 the single clause () -> 0. The clause consists of an argument list
248 (empty), a guard (empty), and a body (which is always a list of expres‐
249 sions) containing the single expression 0. This means that to match out
250 the name and the list of clauses of any function, you'll need to use a
251 pattern like ?Q("'@Name'() -> _@_@Body."), using a dummy clause whose
252 body is a glob lifted one level.
253 To visualize the structure of a syntax tree, you can use the function
255 merl:show(T), which prints a summary. For example, entering
256 merl:show(merl:quote("inc(X, Y) when Y > 0 -> X + Y."))
258 in the Erlang shell will print the following (where the + signs sepa‐
260 rate groups of subtrees on the same level):
261 function: inc(X, Y) when ... -> X + Y.
263 atom: inc
264 +
265 clause: (X, Y) when ... -> X + Y
266 variable: X
267 variable: Y
268 +
269 disjunction: Y > 0
270 conjunction: Y > 0
271 infix_expr: Y > 0
272 variable: Y
273 +
274 operator: >
275 +
276 integer: 0
277 +
278 infix_expr: X + Y
279 variable: X
280 +
281 operator: +
282 +
283 variable: Y
284 This shows another important non-obvious case: a clause guard, even if
286 it's as simple as Y > 0, always consists of a single disjunction of one
287 or more conjunctions of tests, much like a tuple of tuples. Thus:
288 * "when _@Guard ->" will only match a guard with exactly one test
290 * "when _@@Guard ->" will match a guard with one or more comma-sepa‐
292 rated tests (but no semicolons), binding Guard to the list of tests
293 * "when _@_Guard ->" will match just like the previous pattern, but
295 binds Guard to the conjunction subtree
296 * "when _@_@Guard ->" will match an arbitrary nonempty guard, binding
298 Guard to the list of conjunction subtrees
299 * "when _@__Guard ->" will match like the previous pattern, but binds
301 Guard to the whole disjunction subtree
302 * and finally, "when _@__@Guard ->" will match any clause, binding
304 Guard to [] if the guard is empty and to [Disjunction] otherwise
305 Thus, the following pattern matches all possible clauses:
307 "(_@Args) when _@__@Guard -> _@Body"
309
311 default_action() = () -> any():
312 env() = [{Key::id(), pattern_or_patterns()}]:
315 guard_test() = (env()) -> boolean():
318 guarded_action() = switch_action() | {guard_test(), switch_action()}:
321 guarded_actions() = guarded_action() | [guarded_action()]:
324 id() = atom() | integer():
327 location() = erl_anno:location():
330 pattern() = tree() | template():
333 pattern_or_patterns() = pattern() | [pattern()]:
336 switch_action() = (env()) -> any():
339 switch_clause() = {pattern_or_patterns(), guarded_actions()} | {pat‐
342 tern_or_patterns(), guard_test(), switch_action()} | default_ac‐
343 tion():
344 template() = tree() | {id()} | {*, id()} | {template, atom(), term(),
347 [[template()]]}:
348 template_or_templates() = template() | [template()]:
351 text() = string() | binary() | [string()] | [binary()]:
354 tree() = erl_syntax:syntaxTree():
357 tree_or_trees() = tree() | [tree()]:
360
363 alpha(Trees::pattern_or_patterns(), Env::[{id(), id()}]) -> tem‐
364 plate_or_templates()
365 Alpha converts a pattern (renames variables). Similar to
367 tsubst/1, but only renames variables (including globs).
368 See also: tsubst/2.
370 compile(Code) -> term()
372 Equivalent to compile(Code, []).
374 compile(Code, Options) -> term()
376 Compile a syntax tree or list of syntax trees representing a
378 module into a binary BEAM object.
379 See also: compile/1, compile_and_load/2.
381 compile_and_load(Code) -> term()
383 Equivalent to compile_and_load(Code, []).
385 compile_and_load(Code, Options) -> term()
387 Compile a syntax tree or list of syntax trees representing a
389 module and load the resulting module into memory.
390 See also: compile/2, compile_and_load/1.
392 match(Patterns::pattern_or_patterns(), Trees::tree_or_trees()) -> {ok,
394 env()} | error
395 Match a pattern against a syntax tree (or patterns against syn‐
397 tax trees) returning an environment mapping variable names to
398 subtrees; the environment is always sorted on keys. Note that
399 multiple occurrences of metavariables in the pattern is not al‐
400 lowed, but is not checked.
401 See also: switch/2, template/1.
403 meta_template(Templates::template_or_templates()) -> tree_or_trees()
405 Turn a template into a syntax tree representing the template.
407 Meta-variables in the template are turned into normal Erlang
408 variables if their names (after the metavariable prefix charac‐
409 ters) begin with an uppercase character. E.g., _@Foo in the tem‐
410 plate becomes the variable Foo in the meta-template. Further‐
411 more, variables ending with @ are automatically wrapped in a
412 call to merl:term/1, so e.g. _@Foo@ in the template becomes
413 `merl:term(Foo) in the meta-template.
414 print(Ts) -> term()
416 Pretty-print a syntax tree or template to the standard output.
418 This is a utility function for development and debugging.
419 qquote(Text::text(), Env::env()) -> tree_or_trees()
421 Parse text and substitute meta-variables.
423 qquote(StartPos::location(), Text::text(), Env::env()) ->
425 tree_or_trees()
426 Parse text and substitute meta-variables. Takes an initial scan‐
428 ner starting position as first argument.
429 The macro ?Q(Text, Env) expands to merl:qquote(?LINE, Text,
431 Env).
432 See also: quote/2.
434 quote(Text::text()) -> tree_or_trees()
436 Parse text.
438 quote(StartPos::location(), Text::text()) -> tree_or_trees()
440 Parse text. Takes an initial scanner starting position as first
442 argument.
443 The macro ?Q(Text) expands to merl:quote(?LINE, Text, Env).
445 See also: quote/1.
447 show(Ts) -> term()
449 Print the structure of a syntax tree or template to the standard
451 output. This is a utility function for development and debug‐
452 ging.
453 subst(Trees::pattern_or_patterns(), Env::env()) -> tree_or_trees()
455 Substitute metavariables in a pattern or list of patterns,
457 yielding a syntax tree or list of trees as result. Both for nor‐
458 mal metavariables and glob metavariables, the substituted value
459 may be a single element or a list of elements. For example, if a
460 list representing 1, 2, 3 is substituted for var in either of
461 [foo, _@var, bar] or [foo, _@var, bar], the result represents
462 [foo, 1, 2, 3, bar].
463 switch(Trees::tree_or_trees(), Cs::[switch_clause()]) -> any()
465 Match against one or more clauses with patterns and optional
467 guards.
468 Note that clauses following a default action will be ignored.
470 See also: match/2.
472 template(Trees::pattern_or_patterns()) -> template_or_templates()
474 Turn a syntax tree or list of trees into a template or tem‐
476 plates. Templates can be instantiated or matched against, and
477 reverted back to normal syntax trees using tree/1. If the input
478 is already a template, it is not modified further.
479 See also: match/2, subst/2, tree/1.
481 template_vars(Template::template_or_templates()) -> [id()]
483 Return an ordered list of the metavariables in the template.
485 term(Term::term()) -> tree()
487 Create a syntax tree for a constant term.
489 tree(Templates::template_or_templates()) -> tree_or_trees()
491 Revert a template to a normal syntax tree. Any remaining
493 metavariables are turned into @-prefixed atoms or 909-prefixed
494 integers.
495 See also: template/1.
497 tsubst(Trees::pattern_or_patterns(), Env::env()) -> template_or_tem‐
499 plates()
500 Like subst/2, but does not convert the result from a template
502 back to a tree. Useful if you want to do multiple separate sub‐
503 stitutions.
504 See also: subst/2, tree/1.
506 var(Name::atom()) -> tree()
508 Create a variable.
510
512 Richard Carlsson <carlsson.richard@gmail.com>
513 syntax_tools 2.6 merl(3)