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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
Number 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 example:
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 expression 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
remember 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 indicate "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
substitutions, a non-glob variable can be used to inject a sequence of elements, 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 automatically abstracted. For example, the following will increment
the arity 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 syntactical 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 section).
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
expressions) 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 separate 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-separated 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()} | {pattern_or_patterns(), guard_test(),
switch_action()} | default_action():
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()}]) -> template_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 syntax 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 allowed, 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 characters)
begin with an uppercase character. E.g., _@Foo in the template becomes the variable Foo in the
meta-template. Furthermore, 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 scanner 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 debugging.
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 normal 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 templates. 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_templates()
Like subst/2, but does not convert the result from a template back to a tree. Useful if you want
to do multiple separate substitutions.
See also: subst/2, tree/1.
var(Name::atom()) -> tree()
Create a variable.
AUTHORS
Richard Carlsson <carlsson.richard@gmail.com> syntax_tools 2.1.4 merl(3erl)