Data.Text.Prettyprint.Doc.Render.Tutorials.StackMachineTutorial

This module shows how to write a custom prettyprinter backend, based on directly converting a SimpleDoc to an output format using a stack machine. For a tree serialization approach, which may be more suitable for certain output formats, see Data.Text.Prettyprint.Doc.Render.Tutorials.TreeRenderingTutorial.

Rendering to ANSI terminal with colors is an important use case for stack machine based rendering.

The module is written to be readable top-to-bottom in both Haddock and raw source form.

Documentation

The type of available markup

First, we define a set of valid annotations must be defined, with the goal of defining a Doc SimpleHtml. We will later define how to convert this to the output format (Text).

data SimpleHtml

Constructors

Bold

Italics

Color Color

Paragraph

Headline

data Color

Constructors

Red

Green

Blue

Conveinence definitions

bold :: Doc SimpleHtml -> Doc SimpleHtml

italics :: Doc SimpleHtml -> Doc SimpleHtml

paragraph :: Doc SimpleHtml -> Doc SimpleHtml

headline :: Doc SimpleHtml -> Doc SimpleHtml

color :: Color -> Doc SimpleHtml -> Doc SimpleHtml

The rendering algorithm

With the annotation definitions out of the way, we can now define a conversion function from SimpleDoc annotated with our SimpleHtml to the final Text representation.

renderStackMachine :: SimpleDoc SimpleHtml -> StackMachine Builder SimpleHtml ()

The StackMachine type defines a stack machine suitable for many rendering needs. It has two auxiliary parameters: the type of the end result, and the type of the document’s annotations.

Most StackMachine creations will look like this definition: a recursive walk through the SimpleDoc, pushing styles on the stack and popping them off again, and writing raw output.

The equivalent to this in the tree based rendering approach is renderTree.

htmlTag :: SimpleHtml -> (Builder, Builder)

Convert a SimpleHtml annotation to a pair of opening and closing tags. This is where the translation of style to raw output happens.

render :: SimpleDoc SimpleHtml -> Text

We can now wrap our stack machine definition from renderStackMachine in a nicer interface; on successful conversion, we run the builder to give us the final Text, and before we do that we check that the style stack is empty (i.e. there are no unmatched style applications) after the machine is run.

This function does only a bit of plumbing around renderStackMachine, and is the main API function of a stack machine renderer. The tree renderer equivalent to this is render.

Example invocation

We can now render an example document using our definitions:

>>> :set -XOverloadedStrings
>>> import qualified Data.Text.Lazy.IO as TL
>>> :{
>>> let go = TL.putStrLn . render . layoutPretty defaultLayoutOptions
>>> in go (vsep
>>> [ headline "Example document"
>>> , paragraph ("This is a" <+> color Red "paragraph" <> comma)
>>> , paragraph ("and" <+> bold "this text is bold.")
>>> ])
>>> :}
<h1>Example document</h1>
<p>This is a <span style="color: #f00">paragraph</span>,</p>
<p>and <strong>this text is bold.</strong></p>