ajrnz / scemplate

scala template engine and simple expression evaluator

GitHub

scemplate - A scala templating library and expression evaluator

Scemplate is a templating library designed to work well from Scala and to be easy to use. It has features along the lines of other templating libraries such as Jinja, Freemarker, mustache and others but aims to be fairly minimal.

  • SBT: "com.github.ajrnz" %% "scemplate" % "0.5.1"
  • Mill: ivy"com.github.ajrnz::scemplate:0.5.1"

Features

  • A general purpose templating language. Not optimized for any special use case (eg HTML). There are plenty of optimized libraries out there eg ScalaTags

  • Templates are evaluated at runtime and can be modified without using the the compiler. This way you can easily modify external templates without having to changing your code. Content resides in the template where it belongs

  • Missing substitutions are error - no one likes sending/receiving emails of the form "Dear %Firstname%,"

  • Lean - It's about 160k plus fastparse (490k)

  • Easy to use

Templates

There is only one magic character in the templates. That is $. This means to convert a document into a template you only have to replace all $ with the escaped version $$. After that, basic substitutions are similar to standard scala string interpolations. ie. My name is $name or My name is ${name}. Let's look at some examples.

A complete example

Let's say we are doing a simple mail merge. We might define a template:

${person.address.street}
${person.address.town}
${person.address.postCode}

Subject: $subject

Dear ${firstName(person.name)},

As a person ${if age > cutoffAge}over${else}under${endif} $cutoffAge

The full code to define the data and template and to render it is below:

    import ajr.scemplate._
    import ajr.scemplate.implicits._

    case class Address(street: String, town: String, postCode: String)
    case class Person(name: String, age: Int, height: Double, email: String, address: Address)
  
    object Address {
      implicit def toTV(value: Address): TemplateValue = CaseClassEncoder.gen[Address].encode(value)
    }
    object Person {
      implicit def toTV(value: Person): TemplateValue = CaseClassEncoder.gen[Person].encode(value)
    }
  
    val address = Address("1 The Mall", "London", "SW1A 1AA")
    val person = Person("John Doe", 21, 1.76, "john@doe.com", address)
  
    val context = Context()
      .withValues(
        "subject" -> "On offer this week...",
        "person" -> person,
        "cutoffAge" -> 30
      )
      .withFunctions(
        "firstName" -> function(_.asString.split(" ").head)
      )
  
    val templateText =
      """${person.address.street}
        |${person.address.town}
        |${person.address.postCode}
        |
        |Subject: $subject
        |
        |Dear ${firstName(person.name)},
        |
        |As a person ${if person.age>cutoffAge}over${else}under${endif} ${cutoffAge}....
        |""".stripMargin

    val template = new Template(templateText)
    val result = template.render(context)

The result would be the following

1 The Mall
London
SW1A 1AA

Subject: On offer this week...

Dear John,

As a person under 30....

Essentially you provide a template and a context (which a map) and then ask for the template to be rendered. Note that the two implicit def declarations in the Address and Person objects take care of serializing the case class into the context. The serializers are built at compile time and do not use reflection.

A more complicated example

This example shows other language constructs which are available.

${macro renderTran(tran,indent)}
${rep(" ", indent)}${tran.date} ${leftAlign(tran.description,8)} ${rightAlign(formatCurrency(tran.value), 10)}
${endmacro}
$title as at: $date  /  Branch: $branchId
Accounts
${for account in accounts}

Name:    ${account.person.name} (${if account.active}Active${else}Inactive${endif})
Balance: ${formatCurrency(account.balance)}${if account.balance < 0.0} Overdrawn${endif}

${if length(account.transactions) > 0}
Transactions
${for tran in account.transactions}
${renderTran(tran,2)}
${endfor}
${endif}
${endfor}

Note that the functions being called here (leftAlign, rightAlign, formatCurrency, etc) are user defined can be easily defined in scala.

The full source code for these examples can be found in the test's source code.

Language features

The $ character is the single character which activates the template language. Except for simple replacements, the dollar should be followed by braces ie ${...}

Types

All variables in templates are typed. They all have their own wrapper class (like boxing) which derives from a TemplateValue class. The following types exist:

  • String - StringValue
  • Integer - IntValue
  • Double - DoubleValue
  • Boolean - BooleanValue
  • Array - ArrayValue
  • Map - MapValue

Types can be converted to each other via the following methods on TemplateValue

  • .asString
  • .asInt
  • .asDouble
  • .asBoolean

These methods are only valid for conversions which make sense. Inappropriate calls will yield an exception. These methods are also useful when defining functions in scala. Arrays and Maps can be dereferenced using the standard apply (index) notation.

Expressions

Within the template language you can write arbitrary expressions using the following operators

+, -, *, /, %, &&, ||, ==, !=, >, >=, <, <=, (, )

Functions

Functions can be called as in scala using the name(arg, ...) notation. The are added to a context using the withFunctions() method. Make sure you have imported ajr.scemplate.implicits._ beforehand, this will take care of converting regular scala classes to those used by the template renderer. There are a set of helper methods called function which also aid in this process. For example to define a function which repeats a string a number of times you could write the following:

val context = Context()
  .withFunctions("repeat" -> function((str, reps) = str.asString * reps.asInt)

The function's arguments are of type TemplateValue and need to first be converted to their primitive equivalents before being used. The return type can be a standard scala type as the implicits will lift it back into a TemplateValue type automatically.

The defined() function

There is one special function defined(<var>) which can be used to determine whether or not a variable has been declared at. This can be useful if you want to do something based on whether a variable has been defined or not. For example to implement a default value where always having to set the variable would be unidomatic eg. activating a debug flag.

Macros

${macro name(arg1, arg2, ...)}
Text where arg1, arg2, etc take on passed values
${endmacro}

Macros are like functions that are defined in the template. They are called in the same manner as functions. While they have less expressive power than a scala function they often make more sense when they contain a lot of text which really belongs in the template. Macros are only available within the scope in which they are defined.

Conditionals

${if condition}
then text...
${else}
else text
${endif}

The ${else} is optional. The condition can be any conditional expression.

For loops

${for var in list}
<repeated text where $var is defined>
${endif}

list should be an ArrayValue (converted from a scala Seq).

Case classes

Case classes can be put into the context, like any other variable, if a case class encoder has been defined. A case class encoder is just a function which converts the case to a TemplateValue.

    implicit def toTemplateValue(conv: T => TemplateValue) = ...

These functions are usually defined implicitly for convenience. They can be defined automatically at runtime via a macro. If defined in the companion object they will be used automatically where required. Ie:

object MyClass {
  implicit def toTV(value: MyClass): TemplateValue = CaseClassEncoder.gen[MyClass].encode(value)
}

In the template case class members can be accessed via the name they were inserted into the context with and then by the usual dotted notation to access individual members.

Newlines

A single newline (if present) is swallowed at the end of: loop, if/then and macro constructs this makes the templates look more readable. Without this feature a template of the form:

 ${for var in range(0,3)}
 This is line ${var+1}
 ${endif}

would render:

This is line 1

This is line 2

This is line 3

as there are technically two newlines within for construct. You would then need to do the following to get the 'desired' output

 ${for var in range(0,3)}This is line ${var+1}
 ${endif}

If a newline is being removed where it is wanted just insert another one manually.

Expression evaluation

Sometimes it is useful to be able to evaluate expressions dynamically. A helper class is provided to do this: TemplateExpression. It works just like Template except that the ${} characters wrapping the expression are not required. It also means that the return type does not have to be a string. Expressions are evaluated using the eval() method which returns a TemplateValue which can be converted to a primitive type via one of the asXXX methods (see above). Eg:

val tmpl = TemplateExpression("count > (n*n + n)/2")
val greater = tmpl.eval(context).asBoolean

Performance

It's quite fast. Templates are compiled to an AST at runtime and can be reused. They execute very quickly. If you need better performance then you should probably be compiling your templates to scala with something like TWIRL.

The DirectoryRenderer

The directory renderer can be used to render whole directories of templates. For example for app configuration files.

The main static method is renderTree which has the following signature:

  def renderTree(context: Context,
                 reader: DirectoryReader, writer: DirectoryWriter,
                 isTemplate: RelPath => Boolean = _ => true,
                 renamePath: RelPath => RelPath = identity)
  • reader - is a class which generates file and directory paths to be rendered as templates. Both the file path and the file content are substituted
  • writer - is a function which takes a relative path and optional file content (Content of None creates a directory)
  • isTemplate - can be used to specify only certain paths/extensions etc are templates and the others should be copied verbatim
  • renamePath - can be used to modify a path before it is written. For example to remove .tmpl from file names

DirectoryRenderer provides two DirectoryReader implementations:

def fileSystemReader(basePath: Path, ignorePath: RelPath => Boolean = _ => false): DirectoryReader

which reads all files from a basePath with the option to ignore some entries.

And:

def resourceReader(basePath: String, ignorePath: RelPath => Boolean = _ => false, clazz: Class[_] = getClass): DirectoryReader

resourceReader reads entries from resources again with the option to ignore entries. A Class can be provided from which the class loader containing the resources can be found. Note that in order to enumerate resources they need to be either jar or file based.

There is a single implementation of DirectoryWriter:

def fileSystemWriter(basePath: Path, overwrite: Boolean = false): DirectoryWriter

This writes files to the file system below basePath. It will throw an exception if the file already exists unless the overwrite parameter is set to true. Regardless of substitutions or renames, fileSystemWriter will refuse to write files outside basePath.

The Path and RelPath classes used here come from ammonite-ops.

Use:

import ammonite.ops._

to bring the relevant classes and implicits into scope.

Future work

  • Ability to load and merge macros with template
  • Make a number of standard functions available for use within the template
  • Publish for scala.js and scala-native
  • Suggestions welcome...

Change log

0.5.1

  • First release