merl
Metaprogramming in Erlang.
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
or909123
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 variable90901
is a lifted variable,90991
is a glob variable, and9090091
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), bindingGuard
to the list of tests"when _@_Guard ->"
will match just like the previous pattern, but bindsGuard
to the conjunction subtree"when _@_@Guard ->"
will match an arbitrary nonempty guard, bindingGuard
to the list of conjunction subtrees"when _@__Guard ->"
will match like the previous pattern, but bindsGuard
to the whole disjunction subtreeand finally,
"when _@__@Guard ->"
will match any clause, bindingGuard
to[]
if the guard is empty and to[Disjunction]
otherwise
Thus, the following pattern matches all possible clauses:
"(_@Args) when _@__@Guard -> _@Body"
Types
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()
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()
Functions
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()
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()
var(Name::atom()) -> tree()
Create a variable.