merl(3)

📦

version 29.0.6 · 3 versions

💖 merl(3) 💖

Hii so this is the manual page for merl, from the Erlang Module Definition section of the docs, and bestie it is Metaprogramming in Erlang and it is lowkey iconic. Grab a snack, this one's a banger. 🍓

✨ NAME ✨

merl — Metaprogramming in Erlang. That's the whole vibe. 💅

🌈 DESCRIPTION 🌈

Merl is a user-friendly interface to the erl_syntax module, and what it does is make it easy both to build brand-new ASTs from scratch and to match and decompose existing ASTs. Obsessed honestly.

If you need details that are outside the scope of Merl itself, go look at the documentation of erl_syntax. Not my problem, go read it. 🫶

🚀 Quick start

Okay so first thing: to unlock 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)

Bestie. Do you see what happened. The ?Q macros turn the quoted code fragments into ASTs, and they lift metavariables like _@Tuple and _@Number up to the level of your Erlang code, so you can just 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. Periodt. 💅

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. (Yes, that's a big deal. Yes, we love her for it. 👑)

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(...) = ..., the case switches using ?Q(...) patterns, and the 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. For example:

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 list version is a lifesaver when you need to split a long expression over multiple lines. For example:

?Q(["case _@Expr of",
    "  {foo, X} -> f(X);",
    "  {bar, X} -> g(X)",
    "  _ -> h(X)"
"end"])

If there's a syntax error somewhere in the text (like that missing semicolon in the second clause up there, mmhmm, we noticed), this lets Merl generate an error message pointing to the exact line in your source code. So helpful. (Just remember to comma-separate the strings in the list, otherwise Erlang will concatenate the string fragments as if they were a single string. So basically: commas = your best friend, no commas = chaos. 🫠)

🧚 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

After 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

Okay quick heads up bestie: 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 _@__@_. ✨

Finallyyy, 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". And if in addition the name ends with @, as in _@Foo@, then the value of the variable as an Erlang term will be automatically converted to the corresponding abstract syntax tree when it's 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 needing temporary variables in order to inject data, like in:

TmpBar = erl_syntax:abstract(Bar),
Foo = ?Q("{foo, _@TmpBar}")

So convenient. 😭

If the context requires an integer rather than a variable, an atom, or a string, you can't 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 the metavariable:

?Q("'@Name'() -> _@@_." = Function

(note the anonymous glob variable _@@_ to ignore the function body. Sneaky but valid. 🦋)

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). iykyk 💅

🍓 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. Slay. 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})

?Q("{_@@_, _@Last}") = ?Q({a, b, c})

(ignoring all but the last element. Main character energy honestly. 👑)

But bestie, hear me: you cannot have two globs as part of the same sequence. Just don't do it. It's not a vibe.

🌸 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 would not work either. Tragic. 😭

What you have to do in such cases is 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. She's giving placeholder, your honor. 💅

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 is not 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. Wild, right? The tree is deeper than the source code makes it look. 🌈

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

Look at that gorgeous tree. 🌳✨ She's structured, she's layered, she's everything.

okay so this is another important non-obvious case bestie: a clause guard, even if it's literally as simple as Y > 0, is always a single disjunction of one or more conjunctions of tests — it's giving tuple-of-tuples energy fr fr. So:

"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 ✨

now for the type tea ☕💅 every one of these is iconic, no cap:

       default_action() :: fun(() -> any()).

       env() :: [{Key :: id(), pattern_or_patterns()}].

       guard_test() :: fun((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() :: fun((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()].

✨ FUNCTIONS ✨

and now the main event: the functions, bestie. each one is a whole vibe. let's go ✨

alpha

       alpha(pattern_or_patterns(), [{id(), id()}]) ->
                      template_or_templates().

Alpha is the glow-up for a pattern: it renames variables. 💅

It's similar to tsubst/1, but only renames variables (including globs, bestie).

See also: tsubst/2.

compile

       compile(tree_or_trees(), [compile:option()]) -> compile:comp_ret().

Compile a syntax tree (or a list of syntax trees) that represents a module into a binary BEAM object. literally turning your code into a beam file ✨

See also: compile/1, compile_and_load/2.

compile_and_load

       compile_and_load(tree_or_trees(), [compile:option()]) ->
                                 {ok, binary()} |
                                 error |
                                 {error, Errors :: list(), Warnings :: list()}.

Compile a syntax tree or list of syntax trees representing a module, and then load the resulting module into memory. it's giving compile-and-load, slay 💖

See also: compile/2, compile_and_load/1.

match

       match(pattern_or_patterns(), tree_or_trees()) ->
                      {ok, env()} | error.

Match a pattern against a syntax tree (or patterns against syntax trees), and get back an environment mapping variable names to subtrees. the environment is always sorted on keys, bestie. ✨

NOTE: Multiple occurrences of metavariables in the pattern is not allowed, but is not checked. so like, don't do it, but also we won't catch you 👀

See also: switch/2, template/1.

meta_template

       meta_template(template_or_templates()) -> tree_or_trees().

Turn a template into a syntax tree that represents the template. meta af 🦋

Meta-variables in the template get turned into normal Erlang variables if their names (after the metavariable prefix characters) begin with an uppercase character. For example, _@Foo in the template becomes the variable Foo in the meta-template. And also, variables ending with @ are automatically wrapped in a call to merl:term/1, so _@Foo@ in the template becomes merl:term(Foo) in the meta-template. periodt 💅

print

       print(tree_or_trees()) -> ok.

Pretty-print a syntax tree or template to the standard output. it's giving debug glow-up ✨

This is a utility function for development and debugging. so useful bestie.

qquote

       qquote(StartPos :: location(), Text :: text(), Env :: env()) ->
                       tree_or_trees().

Parse text and substitute meta-variables. it's quote but with extra steps, iykyk 🌸

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

       quote(StartPos :: location(), Text :: text()) -> tree_or_trees().

Parse text. that's it, that's the function 💖

Takes an initial scanner starting position as first argument.

The macro ?Q(Text) expands to merl:quote(?LINE, Text).

See also: quote/1.

show

       show(tree_or_trees()) -> ok.

Print the structure of a syntax tree or template to the standard output. it's giving structural tea ☕

This is a utility function for development and debugging. no cap.

subst

       subst(pattern_or_patterns(), env()) -> tree_or_trees().

Substitute metavariables in a pattern or list of patterns, and you get a syntax tree or list of trees as the result. slay ✨

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]. wild, right? 🤯

switch

       switch(tree_or_trees(), [switch_clause()]) -> any().

Match against one or more clauses with patterns and optional guards. it's giving switch energy 💅

Note: clauses following a default action will be ignored. so don't put anything after the default, bestie.

See also: match/2.

template

       template(pattern_or_patterns()) -> template_or_templates().

Turn a syntax tree or list of trees into a template or templates. template-ify it ✨

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. iconic behavior 💖

See also: match/2, subst/2, tree/1.

template_vars

       template_vars(template_or_templates()) -> [id()].

Return an ordered list of the metavariables in the template. getting those vars in order, periodt 📋

term

       term(term()) -> tree().

Create a syntax tree for a constant term. simple, clean, iconic. ✨

tree

       tree(template_or_templates()) -> tree_or_trees().

Revert a template back to a normal syntax tree. un-template-ify it 💅

Any remaining metavariables are turned into @-prefixed atoms or 909-prefixed integers. no cap.

See also: template/1.

tsubst

       tsubst(pattern_or_patterns(), env()) -> template_or_templates().

Like subst/2, but it does not convert the result from a template back to a tree. it's giving tsubst energy ✨

Useful if you want to do multiple separate substitutions. big brain move 🧠

See also: subst/2, tree/1.

var

       var(atom()) -> tree().

Create a variable. that's it, that's the function. periodt 💖

this page was brought to you by Ericsson AB, syntax_tools 4.1, and merl(3). stay iconic, bestie 💅✨

📦 where this page comes from

ubuntu stonking: erlang-doc 1:29.0.6+dfsg-1 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms