Hakyll.Core.Compiler

A Compiler manages targets and dependencies between targets

The most distinguishing property of a Compiler is that it is an Arrow. A compiler of the type Compiler a b is simply a compilation phase which takes an a as input, and produces a b as output.

Compilers are chained using the >>> arrow operation. If we have a compiler

 getResourceString :: Compiler Resource String

which reads the resource, and a compiler

 readPage :: Compiler String (Page String)

we can chain these two compilers to get a

 (getResourceString >>> readPage) :: Compiler Resource (Page String)

Most compilers can be created by combining smaller compilers using >>>.

More advanced constructions are also possible using arrow, and sometimes these are needed. For a good introduction to arrow, you can refer to

http://en.wikibooks.org/wiki/Haskell/Understanding_arrows

A construction worth writing a few paragraphs about here are the require functions. Different variants of this function are exported here, but they all serve more or less the same goal.

When you use only >>> to chain your compilers, you get a linear pipeline -- it is not possible to add extra items from other compilers along the way. This is where the require functions come in.

This function allows you to reference other items, which are then added to the pipeline. Let's look at this crappy ASCII illustration which represents a pretty common scenario:

 read resource >>> pandoc render >>> layout >>> relativize URL's

 @templates/fancy.html@

We want to construct a pipeline of compilers to go from our resource to a proper webpage. However, the layout compiler takes more than just the rendered page as input: it needs the templates/fancy.html template as well.

This is an example of where we need the require function. We can solve this using a construction that looks like:

 ... >>> pandoc render >>> require >>> layout >>> ...
                              |
 @templates/fancy.html@ ------/

This illustration can help us understand the type signature of require.

 require :: (Binary a, Typeable a, Writable a)
         => Identifier
         -> (b -> a -> c)
         -> Compiler b c

Let's look at it in detail:

 (Binary a, Typeable a, Writable a)

These are constraints for the a type. a (the template) needs to have certain properties for it to be required.

 Identifier

This is simply templates/fancy.html: the Identifier of the item we want to require, in other words, the name of the item we want to add to the pipeline somehow.

 (b -> a -> c)

This is a function given by the user, specifying how the two items shall be merged. b is the output of the previous compiler, and a is the item we just required -- the template. This means c will be the final output of the require combinator.

 Compiler b c

Indeed, we have now constructed a compiler which takes a b and produces a c. This means that we have a linear pipeline again, thanks to the require function. So, the require function actually helps to reduce to complexity of Hakyll applications!

Note that require will fetch a previously compiled item: in our example of the type a. It is very important that the compiler which produced this value, produced the right type as well!

Documentation

data Compiler a b

The compiler arrow

Instances

Arrow Compiler

ArrowChoice Compiler

Category Compiler

Functor (Compiler a)

Applicative (Compiler a)

runCompiler

Arguments

:: Compiler () CompileRule

Compiler to run

-> Identifier

Target identifier

-> ResourceProvider

Resource provider

-> [Identifier]

Universe

-> Routes

Route

-> Store

Store

-> Bool

Was the resource modified?

-> Logger

Logger

-> IO (Throwing CompileRule)

Resulting item

Run a compiler, yielding the resulting target and it's dependencies. This version of runCompilerJob also stores the result

getIdentifier :: Compiler a Identifier

Get the identifier of the item that is currently being compiled

getResource :: Compiler a Resource

Get the resource that is currently being compiled

getRoute :: Compiler a (Maybe FilePath)

Get the route we are using for this item

getRouteFor :: Compiler Identifier (Maybe FilePath)

Get the route for a specified item

getResourceString :: Compiler Resource String

Get the resource we are compiling as a string

getResourceLBS :: Compiler Resource ByteString

Get the resource we are compiling as a lazy bytestring

fromDependency :: Identifier -> Compiler a a

Wait until another compiler has finished before running this compiler

require_ :: (Binary a, Typeable a, Writable a) => Identifier -> Compiler b a

Variant of require which drops the current value

require :: (Binary a, Typeable a, Writable a) => Identifier -> (b -> a -> c) -> Compiler b c

Require another target. Using this function ensures automatic handling of dependencies

requireA :: (Binary a, Typeable a, Writable a) => Identifier -> Compiler (b, a) c -> Compiler b c

Arrow-based variant of require

requireAll_ :: (Binary a, Typeable a, Writable a) => Pattern -> Compiler b [a]

Variant of requireAll which drops the current value

requireAll :: (Binary a, Typeable a, Writable a) => Pattern -> (b -> [a] -> c) -> Compiler b c

Require a number of targets. Using this function ensures automatic handling of dependencies

requireAllA :: (Binary a, Typeable a, Writable a) => Pattern -> Compiler (b, [a]) c -> Compiler b c

Arrow-based variant of requireAll

cached :: (Binary a, Typeable a, Writable a) => String -> Compiler Resource a -> Compiler Resource a

unsafeCompiler

Arguments

:: (a -> IO b)

Function to lift

-> Compiler a b

Resulting compiler

Create an unsafe compiler from a function in IO

traceShowCompiler :: Show a => Compiler a a

Compiler for debugging purposes

mapCompiler :: Compiler a b -> Compiler [a] [b]

Map over a compiler

timedCompiler

Arguments

:: String

Message

-> Compiler a b

Compiler to time

-> Compiler a b

Resulting compiler

Log and time a compiler

byExtension

Arguments

:: Compiler a b

Default compiler

-> [(String, Compiler a b)]

Choices

-> Compiler a b

Resulting compiler

Choose a compiler by extension

Example:

 route   "css/*" $ setExtension "css"
 compile "css/*" $ byExtension (error "Not a (S)CSS file")
     [ (".css",  compressCssCompiler)
     , (".scss", sass)
     ]

This piece of code will select the compressCssCompiler for .css files, and the sass compiler (defined elsewhere) for .scss files.