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A type parameter has been added to Doc allowing Strings in the document to be tagged. Tagged strings can then be rendered with the new Pretty.Renderer module, for syntax highlighting.

The new Pretty.Renderer module, also makes it possible to generate non-String output formats, for example HTML.

elm-pretty-printer

A pretty printing library based on 'A Prettier Printer' by Philip Wadler.

This version follows Wadler's paper closely, but is actually ported from a Haskell implementation that is referred to as Wadler/Leijen. Leijen added Column and Nesting constructors in the document type, which make for easier and more flexible indentation.

I have added the ability to have different ways of joining documents when placing them on the same line, or breaking them into multiple lines. When placed within a group , the usual line function will render as a line break or a space; the tightline will render as a line break or "". The separators function allows a string to be given that is placed between docs on the same line, or after the start of the line when placing on multiple lines - useful when rendering commas in a list at the start of the line.

This implementation is usually sufficiently lazy and tail-recursive to perform well under Elm. Occassionally an exponential blow-up can happen if the softline function is not used carefully. Plase raise a GitHub issue on this if it is not performing well enough for you.

Syntax Highlighting and rendering to HTML

The Pretty.Renderer module allows for finer control over how the output is produced.

This requires a Renderer specification to be set up that provides call-back functions that will be used when tagged strings in the document are encountered, and also at line ends.

This is left folded over the output using whatever accumulator type you need in order to transform the output.