A simple templating system with variable substitution and conditionals. The following program illustrates its use:

import Data.Text
import Data.Aeson
import Text.Pandoc.Templates

data Employee = Employee { firstName :: String
                         , lastName  :: String
                         , salary    :: Maybe Int }
instance ToJSON Employee where
  toJSON e = object [ "name" .= object [ "first" .= firstName e
                                       , "last"  .= lastName e ]
                    , "salary" .= salary e ]

employees :: [Employee]
employees = [ Employee "John" "Doe" Nothing
            , Employee "Omar" "Smith" (Just 30000)
            , Employee "Sara" "Chen" (Just 60000) ]

template :: Template
template = either error id $ compileTemplate
  "$for(employee)$Hi, $employee.name.first$. $if(employee.salary)$You make $employee.salary$.$else$No salary data.$endif$$sep$\n$endfor$"

main = putStrLn $ renderTemplate template $ object ["employee" .= employees ]

A slot for an interpolated variable is a variable name surrounded by dollar signs. To include a literal $ in your template, use $$. Variable names must begin with a letter and can contain letters, numbers, _, -, and ..

The values of variables are determined by a JSON object that is passed as a parameter to renderTemplate. So, for example, title will return the value of the title field, and employee.salary will return the value of the salary field of the object that is the value of the employee field.

The value of a variable will be indented to the same level as the variable.

A conditional begins with $if(variable_name)$ and ends with $endif$. It may optionally contain an $else$ section. The if section is used if variable_name has a non-null value, otherwise the else section is used.

Conditional keywords should not be indented, or unexpected spacing problems may occur.

The $for$ keyword can be used to iterate over an array. If the value of the associated variable is not an array, a single iteration will be performed on its value.

You may optionally specify separators using $sep$, as in the example above.