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A Language:Haskell library pretty printing

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The library's central type is Doc for documents. Doc is abstraction over text. The idea is that one maps abstract syntax (modeled by algebraic data types) to docs with the help of combinators serving, for example, horizontal and vertical composition. A doc can then be "shown" literally as text.

Thus:

instance Show Doc 
  where
    -- type is abstract; it can be shown

Documents can be contructed from literals by these combinators:

-- Map string to document
text :: String -> Doc

-- Map int to document
int :: Int -> Doc

Here is an illustration:

> text "hello"
hello
> int 42
42

Documents can be composed in some ways, e.g.:

-- The empty docment
empty :: Doc

-- Compose horizontally
(<>) :: Doc -> Doc -> Doc

-- Compose horizontally with extra space for separation
(<+>) :: Doc -> Doc -> Doc

-- Compose vertically
($$) :: Doc -> Doc -> Doc

Here is an illustration:

> empty

> text "4" <> text "2"
42
> text "before" <+> text "after"
before after
> text "above" $$ text "below"
above
below

The combinators also satisfy some reasonable laws. For example, empty is a unit of horizontal composition -- even the form with an extra space for separation.

> empty <+> int 42
42


Ralf Lämmel edited this article at Wed, 03 May 2017 23:04:59 +0200
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