merl(3erl) Erlang Module Definition merl(3erl)
NAME
merl - Metaprogramming in Erlang.
DESCRIPTION
Metaprogramming in Erlang. Merl is a more user friendly interface to
the erl_syntax module, making it easy both to build new ASTs from
scratch and to match and decompose existing ASTs. For details that are
outside the scope of Merl itself, please see the documentation of
erl_syntax.
Quick start
To enable the full power of Merl, your module needs to include the Merl
header file:
-include_lib("syntax_tools/include/merl.hrl").
Then, you can use the ?Q(Text) macros in your code to create ASTs or
match on existing ASTs. For example:
Tuple = ?Q("{foo, 42}"),
?Q("{foo, _@Number}") = Tuple,
Call = ?Q("foo:bar(_@Number)")
Calling merl:print(Call) will then print the following code:
foo:bar(42)
The ?Q macros turn the quoted code fragments into ASTs, and lifts
metavariables such as _@Tuple and _@Number to the level of your Erlang
code, so you can use the corresponding Erlang variables Tuple and Num-
ber directly. This is the most straightforward way to use Merl, and in
many cases it's all you need.
You can even write case switches using ?Q macros as patterns. For exam-
ple:
case AST of
?Q("{foo, _@Foo}") -> handle(Foo);
?Q("{bar, _@Bar}") when erl_syntax:is_integer(Bar) -> handle(Bar);
_ -> handle_default()
end
These case switches only allow ?Q(...) or _ as clause patterns, and the
guards may contain any expressions, not just Erlang guard expressions.
If the macro MERL_NO_TRANSFORM is defined before the merl.hrl header
file is included, the parse transform used by Merl will be disabled,
and in that case, the match expressions ?Q(...) = ..., case switches
using ?Q(...) patterns, and automatic metavariables like _@Tuple cannot
be used in your code, but the Merl macros and functions still work. To
do metavariable substitution, you need to use the ?Q(Text, Map) macro,
e.g.:
Tuple = ?Q("{foo, _@bar, _@baz}", [{bar, Bar}, {baz,Baz}])
The text given to a ?Q(Text) macro can be either a single string, or a
list of strings. The latter is useful when you need to split a long ex-
pression over multiple lines, e.g.:
?Q(["case _@Expr of",
" {foo, X} -> f(X);",
" {bar, X} -> g(X)",
" _ -> h(X)"
"end"])
If there is a syntax error somewhere in the text (like the missing
semicolon in the second clause above) this allows Merl to generate an
error message pointing to the exact line in your source code. (Just re-
member to comma-separate the strings in the list, otherwise Erlang will
concatenate the string fragments as if they were a single string.)
Metavariable syntax
There are several ways to write a metavariable in your quoted code:
* Atoms starting with @, for example '@foo' or '@Foo'
* Variables starting with _@, for example _@bar or _@Bar
* Strings starting with "'@, for example "'@File"
* Integers starting with 909, for example 9091 or 909123
Following the prefix, one or more _ or 0 characters may be used to in-
dicate "lifting" of the variable one or more levels, and after that, a
@ or 9 character indicates a glob metavariable (matching zero or more
elements in a sequence) rather than a normal metavariable. For example:
* '@_foo' is lifted one level, and _@__foo is lifted two levels
* _@@bar is a glob variable, and _@_@bar is a lifted glob variable
* 90901 is a lifted variable,90991 is a glob variable, and 9090091 is
a glob variable lifted two levels
(Note that the last character in the name is never considered to be a
lift or glob marker, hence, _@__ and 90900 are only lifted one level,
not two. Also note that globs only matter for matching; when doing sub-
stitutions, a non-glob variable can be used to inject a sequence of el-
ements, and vice versa.)
If the name after the prefix and any lift and glob markers is _ or 0,
the variable is treated as an anonymous catch-all pattern in matches.
For example, _@_, _@@_, _@__, or even _@__@_.
Finally, if the name without any prefixes or lift/glob markers begins
with an uppercase character, as in _@Foo or _@_@Foo, it will become a
variable on the Erlang level, and can be used to easily deconstruct and
construct syntax trees:
case Input of
?Q("{foo, _@Number}") -> ?Q("foo:bar(_@Number)");
...
We refer to these as "automatic metavariables". If in addition the name
ends with @, as in _@Foo@, the value of the variable as an Erlang term
will be automatically converted to the corresponding abstract syntax
tree when used to construct a larger tree. For example, in:
Bar = {bar, 42},
Foo = ?Q("{foo, _@Bar@}")
(where Bar is just some term, not a syntax tree) the result Foo will be
a syntax tree representing {foo, {bar, 42}}. This avoids the need for
temporary variables in order to inject data, as in
TmpBar = erl_syntax:abstract(Bar),
Foo = ?Q("{foo, _@TmpBar}")
If the context requires an integer rather than a variable, an atom, or
a string, you cannot use the uppercase convention to mark an automatic
metavariable. Instead, if the integer (without the 909-prefix and
lift/glob markers) ends in a 9, the integer will become an Erlang-level
variable prefixed with Q, and if it ends with 99 it will also be auto-
matically abstracted. For example, the following will increment the ar-
ity of the exported function f:
case Form of
?Q("-export([f/90919]).") ->
Q2 = erl_syntax:concrete(Q1) + 1,
?Q("-export([f/909299]).");
...
When to use the various forms of metavariables
Merl can only parse a fragment of text if it follows the basic syntac-
tical rules of Erlang. In most places, a normal Erlang variable can be
used as metavariable, for example:
?Q("f(_@Arg)") = Expr
but if you want to match on something like the name of a function, you
have to use an atom as metavariable:
?Q("'@Name'() -> _@@_." = Function
(note the anonymous glob variable _@@_ to ignore the function body).
In some contexts, only a string or an integer is allowed. For example,
the directive -file(Name, Line) requires that Name is a string literal
and Line an integer literal:
?Q("-file(\"'@File\", 9090).") = ?Q("-file(\"foo.erl\", 42).")).
This will extract the string literal "foo.erl" into the variable Foo.
Note the use of the anonymous variable 9090 to ignore the line number.
To match and also bind a metavariable that must be an integer literal,
we can use the convention of ending the integer with a 9, turning it
into a Q-prefixed variable on the Erlang level (see the previous sec-
tion).
Globs
Whenever you want to match out a number of elements in a sequence (zero
or more) rather than a fixed set of elements, you need to use a glob.
For example:
?Q("{_@@Elements}") = ?Q({a, b, c})
will bind Elements to the list of individual syntax trees representing
the atoms a, b, and c. This can also be used with static prefix and
suffix elements in the sequence. For example:
?Q("{a, b, _@@Elements}") = ?Q({a, b, c, d})
will bind Elements to the list of the c and d subtrees, and
?Q("{_@@Elements, c, d}") = ?Q({a, b, c, d})
will bind Elements to the list of the a and b subtrees. You can even
use plain metavariables in the prefix or suffix:
?Q("{_@First, _@@Rest}") = ?Q({a, b, c})
or
?Q("{_@@_, _@Last}") = ?Q({a, b, c})
(ignoring all but the last element). You cannot however have two globs
as part of the same sequence.
Lifted metavariables
In some cases, the Erlang syntax rules make it impossible to place a
metavariable directly where you would like it. For example, you cannot
write:
?Q("-export([_@@Name]).")
to match out all name/arity pairs in the export list, or to insert a
list of exports in a declaration, because the Erlang parser only allows
elements on the form A/I (where A is an atom and I an integer) in the
export list. A variable like the above is not allowed, but neither is a
single atom or integer, so '@@Name' or 909919 wouldn't work either.
What you have to do in such cases is to write your metavariable in a
syntactically valid position, and use lifting markers to denote where
it should really apply, as in:
?Q("-export(['@_@Name'/0]).")
This causes the variable to be lifted (after parsing) to the next
higher level in the syntax tree, replacing that entire subtree. In this
case, the '@_@Name'/0 will be replaced with '@@Name', and the /0 part
was just used as dummy notation and will be discarded.
You may even need to apply lifting more than once. To match the entire
export list as a single syntax tree, you can write:
?Q("-export(['@__Name'/0]).")
using two underscores, but with no glob marker this time. This will
make the entire ['@__Name'/0] part be replaced with '@Name'.
Sometimes, the tree structure of a code fragment isn't very obvious,
and parts of the structure may be invisible when printed as source
code. For instance, a simple function definition like the following:
zero() -> 0.
consists of the name (the atom zero), and a list of clauses containing
the single clause () -> 0. The clause consists of an argument list
(empty), a guard (empty), and a body (which is always a list of expres-
sions) containing the single expression 0. This means that to match out
the name and the list of clauses of any function, you'll need to use a
pattern like ?Q("'@Name'() -> _@_@Body."), using a dummy clause whose
body is a glob lifted one level.
To visualize the structure of a syntax tree, you can use the function
merl:show(T), which prints a summary. For example, entering
merl:show(merl:quote("inc(X, Y) when Y > 0 -> X + Y."))
in the Erlang shell will print the following (where the + signs sepa-
rate groups of subtrees on the same level):
function: inc(X, Y) when ... -> X + Y.
atom: inc
+
clause: (X, Y) when ... -> X + Y
variable: X
variable: Y
+
disjunction: Y > 0
conjunction: Y > 0
infix_expr: Y > 0
variable: Y
+
operator: >
+
integer: 0
+
infix_expr: X + Y
variable: X
+
operator: +
+
variable: Y
This shows another important non-obvious case: a clause guard, even if
it's as simple as Y > 0, always consists of a single disjunction of one
or more conjunctions of tests, much like a tuple of tuples. Thus:
* "when _@Guard ->" will only match a guard with exactly one test
* "when _@@Guard ->" will match a guard with one or more comma-sepa-
rated tests (but no semicolons), binding Guard to the list of tests
* "when _@_Guard ->" will match just like the previous pattern, but
binds Guard to the conjunction subtree
* "when _@_@Guard ->" will match an arbitrary nonempty guard, binding
Guard to the list of conjunction subtrees
* "when _@__Guard ->" will match like the previous pattern, but binds
Guard to the whole disjunction subtree
* and finally, "when _@__@Guard ->" will match any clause, binding
Guard to [] if the guard is empty and to [Disjunction] otherwise
Thus, the following pattern matches all possible clauses:
"(_@Args) when _@__@Guard -> _@Body"
DATA TYPES
default_action() = () -> any():
env() = [{Key::id(), pattern_or_patterns()}]:
guard_test() = (env()) -> boolean():
guarded_action() = switch_action() | {guard_test(), switch_action()}:
guarded_actions() = guarded_action() | [guarded_action()]:
id() = atom() | integer():
location() = erl_anno:location():
pattern() = tree() | template():
pattern_or_patterns() = pattern() | [pattern()]:
switch_action() = (env()) -> any():
switch_clause() = {pattern_or_patterns(), guarded_actions()} | {pat-
tern_or_patterns(), guard_test(), switch_action()} | default_ac-
tion():
template() = tree() | {id()} | {*, id()} | {template, atom(), term(),
[[template()]]}:
template_or_templates() = template() | [template()]:
text() = string() | binary() | [string()] | [binary()]:
tree() = erl_syntax:syntaxTree():
tree_or_trees() = tree() | [tree()]:
EXPORTS
alpha(Trees::pattern_or_patterns(), Env::[{id(), id()}]) -> tem-
plate_or_templates()
Alpha converts a pattern (renames variables). Similar to
tsubst/1, but only renames variables (including globs).
See also: tsubst/2.
compile(Code) -> term()
Equivalent to compile(Code, []).
compile(Code, Options) -> term()
Compile a syntax tree or list of syntax trees representing a
module into a binary BEAM object.
See also: compile/1, compile_and_load/2.
compile_and_load(Code) -> term()
Equivalent to compile_and_load(Code, []).
compile_and_load(Code, Options) -> term()
Compile a syntax tree or list of syntax trees representing a
module and load the resulting module into memory.
See also: compile/2, compile_and_load/1.
match(Patterns::pattern_or_patterns(), Trees::tree_or_trees()) -> {ok,
env()} | error
Match a pattern against a syntax tree (or patterns against syn-
tax trees) returning an environment mapping variable names to
subtrees; the environment is always sorted on keys. Note that
multiple occurrences of metavariables in the pattern is not al-
lowed, but is not checked.
See also: switch/2, template/1.
meta_template(Templates::template_or_templates()) -> tree_or_trees()
Turn a template into a syntax tree representing the template.
Meta-variables in the template are turned into normal Erlang
variables if their names (after the metavariable prefix charac-
ters) begin with an uppercase character. E.g., _@Foo in the tem-
plate becomes the variable Foo in the meta-template. Further-
more, variables ending with @ are automatically wrapped in a
call to merl:term/1, so e.g. _@Foo@ in the template becomes
`merl:term(Foo) in the meta-template.
print(Ts) -> term()
Pretty-print a syntax tree or template to the standard output.
This is a utility function for development and debugging.
qquote(Text::text(), Env::env()) -> tree_or_trees()
Parse text and substitute meta-variables.
qquote(StartPos::location(), Text::text(), Env::env()) ->
tree_or_trees()
Parse text and substitute meta-variables. Takes an initial scan-
ner starting position as first argument.
The macro ?Q(Text, Env) expands to merl:qquote(?LINE, Text,
Env).
See also: quote/2.
quote(Text::text()) -> tree_or_trees()
Parse text.
quote(StartPos::location(), Text::text()) -> tree_or_trees()
Parse text. Takes an initial scanner starting position as first
argument.
The macro ?Q(Text) expands to merl:quote(?LINE, Text, Env).
See also: quote/1.
show(Ts) -> term()
Print the structure of a syntax tree or template to the standard
output. This is a utility function for development and debug-
ging.
subst(Trees::pattern_or_patterns(), Env::env()) -> tree_or_trees()
Substitute metavariables in a pattern or list of patterns,
yielding a syntax tree or list of trees as result. Both for nor-
mal metavariables and glob metavariables, the substituted value
may be a single element or a list of elements. For example, if a
list representing 1, 2, 3 is substituted for var in either of
[foo, _@var, bar] or [foo, _@var, bar], the result represents
[foo, 1, 2, 3, bar].
switch(Trees::tree_or_trees(), Cs::[switch_clause()]) -> any()
Match against one or more clauses with patterns and optional
guards.
Note that clauses following a default action will be ignored.
See also: match/2.
template(Trees::pattern_or_patterns()) -> template_or_templates()
Turn a syntax tree or list of trees into a template or tem-
plates. Templates can be instantiated or matched against, and
reverted back to normal syntax trees using tree/1. If the input
is already a template, it is not modified further.
See also: match/2, subst/2, tree/1.
template_vars(Template::template_or_templates()) -> [id()]
Return an ordered list of the metavariables in the template.
term(Term::term()) -> tree()
Create a syntax tree for a constant term.
tree(Templates::template_or_templates()) -> tree_or_trees()
Revert a template to a normal syntax tree. Any remaining
metavariables are turned into @-prefixed atoms or 909-prefixed
integers.
See also: template/1.
tsubst(Trees::pattern_or_patterns(), Env::env()) -> template_or_tem-
plates()
Like subst/2, but does not convert the result from a template
back to a tree. Useful if you want to do multiple separate sub-
stitutions.
See also: subst/2, tree/1.
var(Name::atom()) -> tree()
Create a variable.
AUTHORS
Richard Carlsson <carlsson.richard@gmail.com>
syntax_tools 2.3 merl(3erl)