ginger

{{ "HTML Templates For Humans" }}

Getting Started

Installation

The easiest way to use Ginger in Haskell projects is through cabal:

Stack is also supported; ginger is in Stackage, so it should install just fine. You may find that the version in Stackage lags a bit behind, due to the way Stackage’s LTS releases work. You may want to add Ginger as an extra-deps entry, or even hook the git repository into stack.yml. Please refer to stack documentation for details.

A Basic Template

<!DOCTYPE html>
<html>
    <head>
        <title>{{ title }}</title>
    </head>
    {# This is a comment. Comments are removed from the output. #}
    <body>
        <menu id="nav-main">
        {% for item in navigation %}
            <li><a href="{{ item.url }}">{{ item.label }}</a></li>
        {% endfor %}
        </menu>
        <div class="layout-content-main">
            <h1>{{ title }}</h1>
            {{ body }}
        </div>
    </body>
</html>

Paste this in a file named test.html. Then create another file called test.json, containing this:

We can pass this to the ginger CLI executable, like so:

$ ginger test.html test.json
<!DOCTYPE html>
<html>
    <head>
        <title>Hello, World!</title>
    </head>
    
    <body>
        <menu id="nav-main">
                    <li><a href="/">Home</a></li>
                    <li><a href="https://www.example.com/hello">Example Site</a></li>
                    <li><a href="https://ginger.tobiasdammers.nl/">Ginger Website</a></li>
                </menu>
        <div class="layout-content-main">
            <h1>Hello, World!</h1>
            
        </div>
    </body>
</html>

Now go play with the template (test.html) and the input data (test.json), and see if you can make it do interesting things.

Ginger Syntax: The Basics.

There are two kinds of delimiters. {% ... %} and {{ ... }}. The first one is used to execute statements such as for-loops or assign values, the latter prints the result of an expression to the template. To users of Jinja, Twig, or Django, this should be instantly familiar. If you’re new to these, however, here’s a quick rundown:

  • {{ expression }} takes the value of an expression and injects it into the output at the same position, with automatic HTML-encoding. Ginger has a fully-featured expression language, so things like addition (a + b), property access (a["foo"] or a.foo), etc., will work. You can see this in action in our example template.
  • {% for loopVar in list %} ... {% endfor %} is a loop construct, roughly equivalent to forM in Haskell or foreach in a typical imperative language. Our example uses it to iterate over the navigation variable.
  • {% if expression %} ... {% else %} ... {% endif %} is a conditional, and it works exactly like you’d expect - if expression evaluates to something truthy, the first branch is rendered, otherwise the second branch is rendered.

Haskell Code

So far, we’ve been using the command-line ginger tool, but of course the real beef is using Ginger in a host application. A good way to get an idea of how that works is to look at the code in cli/GingerCLI.hs, the module that defines the ginger command-line tool.

Parsing

The first thing we’ll need to do is to parse some input into a Ginger Template. Templates are polymorphic over the source code position; but this is an implementation you can safely ignore for now. Let’s look at parseGinger:

parseGinger :: forall m. Monad m => IncludeResolver m -> Maybe SourceName -> Source -> m (Either ParserError (Template SourcePos)) parseGinger :: forall m. Monad m => IncludeResolver m -> Maybe SourceName -> Source -> m (Either ParserError (Template SourcePos))

First of all, this function is parametric over m, a Monad instance in which the whole thing will run. We would need this to load includes, but since we’re not going to do that for now, we will pick Identity.

IncludeResolver m is a synonym for SourceName -> m (Maybe Source), but since we won’t be handling includes yet, we can use (const $ return Nothing): we simply fail to load any and all includes.

The Maybe SourceName parameter is used to display source file names in error messages; we’ll pass Nothing, because we don’t have one.

The Source argument is the actual source code, and it’s currently a synonym for String.

Armed with all this, we can write the parsing part:

Running - The Easy Interface

Ginger’s “easy” interface is rather simple, offering just two flavors of a template rendering function, monadic (easyRenderM) and effect-less (easyRender). Using them is as simple as it gets, and a simple example should make things clear:

Note that, because Ginger is a dynamically typed language (or, actually, an untyped language), passing values to a template execution context requires conversion to Ginger values, represented on the Haskell side as the GVal type. The ToGVal typeclass exposes a number of convenience functions that can be used for this purpose, and there are instances for many of Haskell’s standard data types.

Running - The Raw Interface

The raw interface gives you more control of the context passing, output writing, encoding, etc.

The core function for running a template is ‘runGinger’ (or its monadic flavor ‘runGingerT’); in order to pass an initial context to the template engine, pass a suitable ‘GingerContext’, which you can create using the ‘makeContext’ / ‘makeContextM’ functions.

An example call (for running a template in ‘IO’) would look something like this:

To dissect it:

  • scopeLookup should be a function that takes a Text key and returns a suitable GVal into the Run monad. This is slightly more generic than passing in the entire context at once, as it allows you to load values on demand, rather than having all the values ready upfront.
  • The putStr . Text.unpack . htmlSource part is how you want Ginger to implement output writing, that is, this function is called once for every bit of output the template generates. In this example, we’re getting the raw HTML source out, convert it to a String, and print it on stdout - but in a web application, we might instead send it to the HTTP response, and in a static site generator, we’d write it to a file.
  • tpl is the template we want to run.

A Worked Example

{-# LANGUAGE OverloadedStrings #-}
module Main where

import Data.HashMap.Strict (fromList, HashMap)
import qualified Data.HashMap.Strict as HashMap
import Data.Hashable
import Data.Text (Text)
import System.Exit (exitFailure)
import System.IO (IOMode(ReadMode), openFile, hGetContents)
import System.IO.Error (tryIOError)
import Text.Ginger
       (makeContextHtml, Template, toGVal, runGinger, parseGingerFile, VarName)
import Text.Ginger.GVal (ToGVal, GVal)
import Text.Ginger.Html (htmlSource)


-- A simple hashmap that we'll use as our template context
sampleContext :: HashMap Text Text
sampleContext = fromList [("name", "Alice")]


-- Given a Template and a HashMap of context, render the template to Text
render :: Template -> HashMap VarName Text -> Text
render template contextMap =
  let contextLookup = flip scopeLookup contextMap
      context = makeContextHtml contextLookup
  in htmlSource $ runGinger context template


-- Wrapper around HashMap.lookup that applies toGVal to the value found.
-- Any value referenced in a template, returned from within a template, or used
-- in a template context, will be a GVal
scopeLookup
  :: (Hashable k, Eq k, ToGVal m b)
  => k -> HashMap.HashMap k b -> GVal m
scopeLookup key context = toGVal $ HashMap.lookup key context


loadFileMay :: FilePath -> IO (Maybe String)
loadFileMay fn =
  tryIOError (loadFile fn) >>= \e ->
    case e of
      Right contents -> return (Just contents)
      Left _ -> return Nothing

  where
    loadFile :: FilePath -> IO String
    loadFile fn' = openFile fn' ReadMode >>= hGetContents


-- Assuming there's an html file called "base.html" in the current directory and
-- that html file's contents are `Hi, {{ name }}`, attempt to parse "base.html"
-- and print the rendered template
-- >>> run
-- "Hi, Alice"
main :: IO ()
main = do
  template <- parseGingerFile loadFileMay "base.html"
  case template of
    Left err -> print err >> exitFailure
    Right template' -> print $ render template' sampleContext

Further Reading

For full documentation on Ginger’s Haskell API, check out the Haddock documentation that ships with it, which is also available on Hackage: https://hackage.haskell.org/package/ginger