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ErlyDTL compiles Django Template Language to Erlang bytecode.

Project homepage:

ErlyDTL implements the Django Template Language as documented for version 1.6, here:

Despite our best efforts to be completely compatible with the Django Template Languge, there are still a few differences.

Call for maintainers

My current position in life does not give me the time I feel this project deserves. So, if you feel that there are things that ought to be done, and have the skills and time to do them, please get in touch. -- @kaos

The erlydtl branches & tags

We follow a gitflow inspired branching model, where master is the latest released version (a.k.a. stable), and use develop for stuff that may break from time to time.

master branch

Releases are made from the master branch, with dependency versions pinned down and a hard coded version number in the app file.

develop branch

This is were all the action is, and at times may be slightly broken. Suitable for early adopters who aren't afraid of a little debugging and hopefully also reporting issues.

release tags

Whenever master is deemed stable with a nice set of additions/changes, it is tagged for a new release.

As we're still going for the big 1.0 release, breaking changes may be introduced also on minor release bumps; but after that, we'll stick with semver.


To compile ErlyDTL, run


in this directory.

Do not use Erlang R16B03

The erl syntax tools is broken in Erlang R16B03, use R16B03-1 or any other supported version instead.

Do use a recent version of rebar

In case of compilation issues, make sure your version of rebar is up-to-date.

Older versions of rebar does not support the raw option for dependencies, and may produce an error message like this:

ERROR: Invalid dependency specification {merl,".*",

Template compilation


erlydtl:compile_file("/path/to/template.dtl", my_module_name)

erlydtl:compile_file("/path/to/template.dtl", my_module_name, Options)

erlydtl:compile_template("<html>{{ foo }}</html>", my_module_name)

erlydtl:compile_template("<html>{{ foo }}</html>", my_module_name, Options)


{ok, Module}
{ok, Module, Warnings}
{ok, Module, Binary}
{ok, Module, Binary, Warnings}

{error, Errors, Warnings}

Options is a proplist possibly containing:

  • auto_escape - Control automatic HTML escaping of template values. Enabled by default.

  • binary - Include the compiled template binary code in the result tuple (between the module name and any warning/error lists). Note, this option is named the same as for the Erlang compiler, with similar use, except that this option does NOT affect whether or not a .beam file is saved.

  • binary_strings - Whether to compile strings as binary terms (rather than lists). Defaults to true.

  • compiler_options - Proplist with extra options passed directly to compiler:forms/2. This can prove useful when using extensions to add extra defines etc when compiling the generated code.

  • constants - Replace template variables with a constant value when compiling the template. This can not be overridden when rendering the template. See also default_vars.

  • custom_filters_modules deprecated - A list of modules to be used for handling custom filters. The modules will be searched in order and take precedence over the built-in filters. Each custom filter should correspond to an exported filter, e.g.

    some_filter(Value) -> iolist()

    If the filter takes any arguments (e.g. "foo:2"), those will be added to the call:

    some_filter(Value, Arg) -> iolist()
  • custom_tags_dir - Directory of DTL files (no extension) includable as tags. E.g. if $custom_tags_dir/foo contains <b>{{ bar }}</b>, then {% foo bar=100 %} will evaluate to <b>100</b>.

  • custom_tags_modules deprecated - A list of modules to be used for handling custom tags. The modules will be searched in order and take precedence over custom_tags_dir. Each custom tag should correspond to an exported function with one of the following signatures:

    some_tag(TagVars)          -> iolist()
    some_tag(TagVars, Options) -> iolist()

    The TagVars are variables provided to a custom tag in the template's body (e.g. {% foo bar=100 %} results in TagVars = [{bar, 100}]). The Options are options passed as the second argument to the render/2 call at render-time. (These may include any options, not just locale and translation_fun.)

  • debug_compiler - Enable compiler debug diagnostics. Currently it debug prints the options passed to compile:forms (i.e. if verbosity is >= 2; that is, with two or more verbose options) and enables the saving of the compiled template in source form to a .erl file.

  • debug_info - This option is passed to compile:forms to include debug information in the compiled module.

  • debug_root - Only applies when debug_compiler is true. The root directory for debug source dumps. If set to false, no source dump will be saved. Defaults to undefined, leaving the source dump next to the source template file.

  • default_libraries - A list of libraries that should be loaded by default when compiling a template. Libraries can be specified either by name (when there is a name to module mapping also provided in the libraries option) or by module.

  • default_vars - Provide default values for variables. Any value from the render variables takes precedence. Notice: in case the value is a fun/0, it will be called at compile time. See also constants.

  • doc_root - Included template paths will be relative to this directory; defaults to the compiled template's directory.

  • extension_module experimental - This is work in progress to make erlydtl extensible.

  • force_recompile - Recompile the module even if the source's checksum has not changed. Useful for debugging.

  • libraries - A list of {Name, Module} libraries implementing custom tags and filters. Module should implement the erlydtl_library behaviour (see [Custom tags and filters] below).

  • lists_0_based - Compatibility warning Defaults to false, giving 1-based list access, as is common practice in Erlang. Set it to true to get 1-based access as in Django, or to defer to not decide until render time, using the render option lists_0_based. See also tuples_0_based.

  • locale - Locale to translate to during compile time. May be specified multiple times as well as together with the locales option.

  • locales - A list of locales to be passed to translation_fun. Defaults to [].

  • no_env - Do not read additional options from the OS environment variable ERLYDTL_COMPILER_OPTIONS.

  • no_load - Do not load the compiled template.

  • out_dir - Directory to store generated .beam files. If not specified, no .beam files will be created and a warning is emitted. To silence the warning, use {out_dir, false}.

  • reader - {module, function} tuple that takes a path to a template, may be ReaderOptions and returns a binary with the file contents. Defaults to {file, read_file}. Useful for reading templates from a network resource.

  • reader_options - list of {option_name, Option} that passed as the second parameter to reader.

      extra_reader(FileName, ReaderOptions) ->
       UserID = proplists:get_value(user_id, ReaderOptions, <<"IDUnknown">>),
       UserName = proplists:get_value(user_name, ReaderOptions, <<"NameUnknown">>),
       case file:read_file(FileName) of
        {ok, Data} when UserID == <<"007">>, UserName == <<"Agent">> ->
         {ok, Data};
        {ok, _Data} ->
         {error, "Not Found"};
        Err ->
      CompileResult = erlydtl:compile(Body, ModName,
      										  {reader, {?MODULE, extra_reader}},
      										  {reader_options, [{user_id, <<"007">>},
      											    {user_name, <<"Agent">>}]}
  • record_info - List of records to look for when rendering the template. Each record info is a tuple with the fields of the record:

    {my_record, record_info(fields, my_record)}
  • return - Short form for both return_warnings and return_errors.

  • return_warnings - If this flag is set, then an extra field containing warnings is added to the tuple returned on success.

  • return_errors - If this flag is set, then an error-tuple with two extra fields containing errors and warnings is returned when there are errors.

  • report - Short form for both report_warnings and report_errors.

  • report_warnings - Print warnings as they occur.

  • report_errors - Print errors as they occur.

  • translation_fun - A two-argument fun to use for translating blocktrans blocks, trans tags and _(..) expressions at compile time. This will be called once for each pair of translated element and locale specified with locales and locale options. The fun should take the form:

    fun (Block::string(), Locale|{Locale, Context}) ->
        <<"ErlyDTL code">>::binary() | default
      when Locale::string(), Context::string().

    Please, keep in mind, that if your templates where not specially designed, you probably still need render time translations. See description of the translation_fun render option for more details on the translation context.

    Notice, you may instead pass a fun/0, {Module, Function} or {Module, Function, Args} which will be called recursively until it yields a valid translation function, at which time any needed translation setup actions can be carried out prior to returning the next step (either another setup function/tuple, or the translation function).

    %% sample translation setup
    fun () ->
        fun translation_engine:translate/2
  • tuples_0_based - Compatibility warning Defaults to false, giving 1-based tuple access, as is common practice in Erlang. Set it to true to get 1-based access as in Django, or to defer to not decide until render time, using the render option tuples_0_based. See also lists_0_based.

  • vars deprecated - Use default_vars instead. Variables (and their values) to evaluate at compile-time rather than render-time.

  • verbose - Enable verbose printing of compilation progress. Add several for even more verbose (e.g. debug) output.

  • w - Enable/Disable compile time checks.

    Available checks:

    • non_block_tag indicated that there is other data than block tags in an extends-template (e.g. a template that begins with the extends tag).
  • warnings_as_errors - Treat warnings as errors.

Helper compilation

Helpers provide additional templating functionality and can be used in conjunction with the custom_tags_module option above. They can be created from a directory of templates thusly:

erlydtl:compile_dir("/path/to/dir", my_helper_module_name)

erlydtl:compile_dir("/path/to/dir", my_helper_module_name, Options)

The resulting module will export a function for each template appearing in the specified directory. Options is the same as for compile/3.

Compiling a helper module can be more efficient than using custom_tags_dir because the helper functions will be compiled only once (rather than once per template).

Notice: The exported template functions return an iolist() on success only, failures are non-local (e.g. as a throw). To get the result in wrapped tuple {ok, iolist()} | {error, Reason} call one of the render functions: render(Tag) | render(Tag, Vars) | render(Tag, Vars, Opts).

Usage (of a compiled template)


my_compiled_template:render(Variables) -> {ok, IOList} | {error, Err}

Variables is a proplist, dict, gb_tree, or a parameterized module (whose method names correspond to variable names). The variable values can be atoms, strings, binaries, or (nested) variables.

IOList is the rendered template.


my_compiled_template:render(Variables, Options) -> {ok, IOList} | {error, Err}

Same as render/1, but with the following options:

  • translation_fun - A fun/1 or fun/2 that will be used to translate strings appearing inside {% trans %} and {% blocktrans %} tags at render-time. The simplest TranslationFun would be fun(Val) -> Val end. Placeholders for blocktrans variable interpolation should be wrapped in {{ and }}. In case of fun/2, the extra argument is the current locale, possibly together with a translation context in a tuple:

    fun (Val|{Val, {Plural_Val, Count}}, Locale|{Locale, Context}) ->

    The context is present when specified in the translation tag. Example:

    {% trans "Some text to translate" context "app-specific" %}
    {% blocktrans context "another-context" %}
      Translate this for {{ name }}.
    {% endblocktrans %}

    The plural form is present when using count and plural in a blocktrans block:

    {% blocktrans count counter=var|length %}
      There is {{ counter }} element in the list.
    {% plural %}
      There are {{ counter }} elements in the list.
    {% endblocktrans %}

    Render time translation function is also used to translate dates. Date tokens mimics those used in django, so you may reuse django translations. Tokens may appear in a date are:

    1. Full months names, capitalized ("January" .. "December");
    2. 3 letters months names ("jan" .. "dec");
    3. Associated Press style months ("Jan." .. "Dec." with "abbrev.month" context);
    4. Alternative month name, for "E" option ("January" .. "December" with "alt. month" context);
    5. Full week day names, capitalized ("Monday" .. "Sunday");
    6. Abbreviated week day names, capitalized ("Mon" .. "Sun");
    7. day time tokens ("AM", "PM", "a.m.", "p.m.", "noon", "midnight");

    While date token values may be passed as lists, consider using binaries as a default string format. Here is a simple but robust translation function stub:

       translation_placeholder({Val,{Plural, Count}}, {L, C}) when is_list(Val) ->
         translation_placeholder({list_to_binary(Val),{Plural, Count}}, LC);
       translation_placeholder(Val, {L, C}) when is_list(Val) ->
         translation_placeholder(list_to_binary(Val), LC);
       translation_placeholder(Val, {L, C}) when is_list(C) ->
         translation_placeholder(Val, list_to_binary(C));
       translation_placeholder(Val, {L, C}) when is_list(C) ->
         io:format("Translating ~p into ~p with context ~p~n", [Val, L, C]),
         %% do nothing and return original value

    The translation fun can also be a fun/0, {Module, Function} or {Module, Function, Args} which will be called recursively until it yields a valid translation function, at which time any needed translation setup actions can be carried out prior to returning the next step (either another setup function/tuple, or the translation function).

  • lists_0_based - If the compile option lists_0_based was set to defer, pass this option (or set it to true, {lists_0_based, true}) to get 0-based list indexing when rendering the template. See also tuples_0_based.

  • locale - A string specifying the current locale, for use with the translation_fun compile-time option.

  • tuples_0_based - If the compile option tuples_0_based was set to defer, pass this option (or set it to true, {tuples_0_based, true}) to get 0-based tuple indexing when rendering the template. See also lists_0_based.


my_compiled_template:translatable_strings() -> [String]

List of strings appearing in {% trans %} and _(..) tags.


my_compiled_template:translated_blocks() -> [String]

List of strings appearing in {% blocktrans %}...{% endblocktrans %} blocks; the translations (which can contain ErlyDTL code) are hard-coded into the module and appear at render-time. To get a list of translatable blocks before compile-time, use the provided blocktrans_extractor module.


my_compiled_template:source() -> {FileName, CheckSum}

Name and checksum of the original template file.


my_compiled_template:dependencies() -> [{FileName, CheckSum}]

List of names/checksums of templates included by the original template file. Useful for frameworks that recompile a template only when the template's dependencies change.


my_compiled_template:variables() -> [Variable::atom()]

Sorted list of unique variables used in the template's body. The list can be used for determining which variable bindings need to be passed to the render/3 function.


my_compiled_template:default_variables() -> [Variable::atom()]

Like variables/0, but for any variable which have a default value provided at compile time.


my_compiled_template:constants() -> [Variable::atom()]

Like default_variables/0, but these template variables has been replaced with a fixed value at compile time and can not be changed when rendering the template.

Custom tags and filters

Starting with release 0.9.1, the recommended way to add custom tags and filters are to register a module implementing the erlydtl_library behaviour. There are two functions needed to implement a custom library: version/0 and inventory/1.

The version/0 function is to be able to keep backwards compatibility in face of an evolving library behaviour, and should return the version of the behaviour the library supports. The valid range of versions is in the spec for the version/0 callback in erlydtl_library.erl.

Library version 1

The inventory/1 function is called with either filters or tags as argument, and should return a list of all filters or tags available in that library, respectively (see spec in erlydtl_library.erl for details on syntax for this list).

Tag functions must take a list of arguments, and may take a list of render options, and should return an iolist() as result value (see custom_tags_modules compile option).

Filter functions must take an iolist() value to filter, and may take an argument, and should return an iolist() as result value (see custom_filters_modules compile option).

Differences from standard Django Template Language

  • csrf_token The Cross Site Request Forgery tag is not implemented.
  • url The url tag is not implemented. This should be addressed in a future release.
  • List indexing is 1-based in erlydtl, while 0-based in Django (see #156).
  • For an up-to-date list, see all issues marked dtl_compat.
  • Erlang specifics: Template variables may be prefixed with underscore (_) to avoid "unused variable" warnings (see #164).
  • cycle tags do not support independently moving the cycle value from the original loop.


From a Unix shell, run:

make tests

Note that the tests will create some output in tests/output in case of regressions.


ErlyDTL is released under the MIT license.