Text.Mustache

How to use this library

This module exposes some of the most convenient functions for dealing with mustache templates.

Compiling with automatic partial discovery

The easiest way of compiling a file and its potential includes (called partials) is by using the automaticCompile function.

main :: IO ()
main = do
  let searchSpace = [".", "./templates"]
      templateName = "main.mustache"

  compiled <- automaticCompile searchSpace templateName
  case compiled of
    Left err -> print err
    Right template -> pure () -- this is where you can start using it

The searchSpace encompasses all directories in which the compiler should search for the template source files. The search itself is conducted in order, from left to right.

Should your search space be only the current working directory, you can use localAutomaticCompile.

The templateName is the relative path of the template to any directory of the search space.

automaticCompile not only finds and compiles the template for you, it also recursively finds any partials included in the template as well, compiles them and stores them in the partials hash attached to the resulting template.

The compiler will throw errors if either the template is malformed or the source file for a partial or the template itself could not be found in any of the directories in searchSpace.

Substituting

In order to substitute data into the template it must be an instance of the ToMustache typeclass or be of type Value.

This libray tries to imitate the API of aeson by allowing you to define conversions of your own custom data types into Value, the type used internally by the substitutor via typeclass and a selection of operators and convenience functions.

Example

  data Person = { age :: Int, name :: String }

  instance ToMustache Person where
    toMustache person = object
      [ "age" ~> age person
      , "name" ~> name person
      ]

The values to the left of the ~> operator has to be of type Text, hence the OverloadedStrings can becomes very handy here.

Values to the right of the ~> operator must be an instance of the ToMustache typeclass. Alternatively, if your value to the right of the ~> operator is not an instance of ToMustache but an instance of ToJSON you can use the ~= operator, which accepts ToJSON values.

  data Person = { age :: Int, name :: String, address :: Address }

  data Address = ...

  instance ToJSON Address where
    ...

  instance ToMustache Person where
    toMustache person = object
      [ "age" ~> age person
      , "name" ~> name person
      , "address" ~= address person
      ]

All operators are also provided in a unicode form, for those that, like me, enjoy unicode operators.

Manual compiling

You can compile templates manually without requiring the IO monad at all, using the compileTemplate function. This is the same function internally used by automaticCompile and does not check if required partial are present.

More functions for manual compilation can be found in the Text.Mustache.Compile module. Including helpers for finding lists of partials in templates.

Additionally the compileTemplateWithCache function is exposed here which you may use to automatically compile a template but avoid some of the compilation overhead by providing already compiled partials as well.

Fundamentals

This library builds on three important data structures/types.

Value

A data structure almost identical to Data.Aeson.Value extended with lambda functions which represents the data the template is being filled with.

ToMustache

A typeclass for converting arbitrary types to

Value

, similar to Data.Aeson.ToJSON but with support for lambdas.

Template

Contains the

STree

, the syntax tree, which is basically a list of text blocks and mustache tags. The

name

of the template and its

partials

cache.

Compiling

During the compilation step the template file is located, read, then parsed in a single pass (compileTemplate), resulting in a Template with an empty partials section.

Subsequenty the STree of the template is scanned for included partials, any present TemplateCache is queried for the partial (compileTemplateWithCache), if not found it will be searched for in the searchSpace, compiled and inserted into the template's own cache as well as the global cache for the compilation process.

Internally no partial is compiled twice, as long as the names stay consistent.

Once compiled templates may be used multiple times for substitution or as partial for other templates.

Partials are not being embedded into the templates during compilation, but during substitution, hence the partials cache is vital to the template even after compilation has been completed. Any non existent partial in the cache will rsubstitute to an empty string.

Substituting