heterocephalus
A type-safe template engine for working with front end development tools
https://github.com/arowM/heterocephalus#readme
LTS Haskell 22.43: | 1.0.5.7 |
Stackage Nightly 2024-12-07: | 1.0.5.7 |
Latest on Hackage: | 1.0.5.7 |
heterocephalus-1.0.5.7@sha256:11350453cf7b9c9e4a945c57d9e7653995cb3d64f3b4307df1b7ce9f8808dfd5,2972
Module documentation for 1.0.5.7
Heterocephalus template engine
A type-safe template engine for working with popular front end development tools.
Any PRs are welcome, even for documentation fixes. (The main author of this library is not an English native.)
Who should use this?
If you are planning to use Haskell with recent web front-end tools like gulp, webpack, npm, etc, then this library can help you!
There are many Haskell template engines today. Shakespeare is great because it checks template variables at compile time. Using Shakespeare, it’s not possible for the template file to cause a runtime-error.
Shakespeare provides its own original ways of writing HTML
(Hamlet),
CSS
(Cassius
/
Lucius),
and JavaScript
(Julius).
If you use these original markup languages, it is possible to use control
statements like forall
(for looping), if
(for conditionals), and case
(for case-splitting).
However, if you’re using any other markup language (like
pug, slim,
haml, normal HTML, normal CSS, etc), Shakespeare only
provides you with the
Text.Shakespeare.Text
module. This gives you variable interpolation, but no control statements like
forall
, if
, or case
.
Haiji
is another interesting
library. It has all the features we require, but its templates take a very
long time to compile with
GHC >= 7.10.
Heterocephalus fills this missing niche. It gives you variable interpolation along with control statements that can be used with any markup language. Its compile times are reasonable.
Features
Here are the main features of this module.
-
DO ensure that all interpolated variables are in scope
-
DO ensure that all interpolated variables have proper types for the template
-
DO expand the template literal on compile time
-
DO provide a way to use
forall
,if
, andcase
statments in the templateText.Shakespeare.Text.text
has a way to do variable interpolation, but no way to use these types of control statements. -
DO NOT enforce that templates obey a peculiar syntax
Shakespeare templates make you use their original style (Hamlet, Cassius, Lucius, Julius, etc). The
Text.Shakespeare.Text.text
function does not require you to use any particular style, but it does not have control statements likeforall
,if
andcase
.This makes it impossible to use Shakespeare with another template engine such as
pug
in front end side. It is not suitable for recent rich front end tools. -
DO NOT have a long compile time
haiji
is another awesome template library. It has many of our required features, but it takes too long to compile when used with ghc >= 7.10. -
DO NOT provide unneeded control statements
Other template engines like EDE provide rich control statements like importing external files. Heterocephalus does not provide control statements like this because it is meant to be used with a rich front-end template engine (like pug, slim, etc).
Usage
You can compile external template files with the following four functions:
compileTextFile
: A basic function that embeds variables without escaping and without default values.compileTextFileWithDefault
: Same ascompileTextFile
but you can set default template values.compileHtmlFile
: Same ascompileTextFile
but all embeded variables are escaped for html.compileHtmlFileWithDefault
: Same ascompileHtmlFile
but you can set default template values.
For more details, see the latest haddock document.
Checking behaviours in ghci
To check the behaviour, you can test in ghci
as follows. Note that
compileText
and compileHtml
are used for checking syntaxes.
$ stack install heterocephalus # Only first time
$ stack repl --no-build --no-load
Prelude> :m Text.Heterocephalus Text.Blaze.Renderer.String
Prelude> :set -XTemplateHaskell -XQuasiQuotes
Prelude> let a = 34; b = "<script>"; in renderMarkup [compileText|foo #{a} #{b}|]
"foo 34 <script>"
Prelude> let a = 34; b = "<script>"; in renderMarkup [compileHtml|foo #{a} #{b}|]
"foo 34 <script>"
Syntax
The Text.Heterocephalus module provides two major features for use in template files: variable interpolation and control statements.
Variable interpolation
A Haskell variable can be embedded in the template file with the #{foo}
syntax. The value of the variable will be injected in at run time.
Basic usage
All of following are correct (this assumes that you have already declared the
var
variable in your Haskell program and it is in scope):
#{ var }
#{var}
#{ var}
#{var }
#{ var }
The variable must be an instance of
Text.Blaze.ToMarkup
.
Advanced usage
You can use functions and data constructors as well.
#{ even num }
#{ num + 3 }
#{ take 3 str }
#{ maybe "" id (Just b) }
Control statements
Only two type of control statements are provided.
Forall
%{ forall x <- xs }
#{x}
%{ endforall }
%{ forall (k,v) <- kvs }
#{k}: #{v}
%{ endforall }
If
%{ if even num }
#{num} is even number.
%{ else }
#{num} is odd number.
%{ endif }
%{ if (num < 30) }
#{ num } is less than 30.
%{ elseif (num <= 60) }
#{ num } is between 30 and 60.
%{ else }
#{ num } is over 60.
%{ endif }
Case
%{ case maybeNum }
%{ of Just 3 }
num is 3.
%{ of Just num }
num is not 3, but #{num}.
%{ of Nothing }
num is not anything.
%{ endcase }
%{ case nums }
%{ of (:) n _ }
first num is #{n}.
%{ of [] }
no nums.
%{ endcase }
Why we do not provide maybe
and with
?
TODO
Discussions about this topic is on issue #9.
Why “heterocephalus”?
“Heterocephalus” is the scientific name of the naked mole-rat.
Changes
Change Log
Version 1.0.5.7 (2022-07-13)
Support GHC 9.2
Version 1.0.5.0 (2017-06-05)
New features
- Add settings to be able to change the character used to deliminate control statements #18
Document updates
- Fixed small spelling/grammars on readme #19
Version 1.0.4.0 (2017-02-07)
New features
- Expose htmlSetting and textSetting
Version 1.0.3.0 (2017-01-24)
New features
- Add
case
control statement
Version 1.0.2.0 (2016-12-13)
New features
- Add
compileTextFileWith
andcompileHtmlFileWith
to inject extra variables - Add
ScopeM
type for specifying extra template variables - Add
setDefault
andoverwrite
for constructingScopeM