Text.Parsec.Indent

Documentation

A module to construct indentation aware parsers. Many programming language have indentation based syntax rules e.g. python and Haskell. This module exports combinators to create such parsers.

The input source can be thought of as a list of tokens. Abstractly each token occurs at a line and a column and has a width. The column number of a token measures is indentation. If t1 and t2 are two tokens then we say that indentation of t1 is more than t2 if the column number of occurrence of t1 is greater than that of t2.

Currently this module supports two kind of indentation based syntactic structures which we now describe:

Block

A block of indentation c is a sequence of tokens with indentation at least c. Examples for a block is a where clause of Haskell with no explicit braces.

Line fold

A line fold starting at line l and indentation c is a sequence of tokens that start at line l and possibly continue to subsequent lines as long as the indentation is greater than c. Such a sequence of lines need to be folded to a single line. An example is MIME headers. Line folding based binding separation is used in Haskell as well.

Types

type IndentParser s u a = ParsecT s u (State SourcePos) a

Indentation sensitive parser type. Usually m will be Identity as with any ParsecT

runIndent :: SourceName -> State SourcePos a -> a

Run the result of an indentation sensitive parse

Blocks

withBlock :: Stream s (State SourcePos) z => (a -> [b] -> c) -> IndentParser s u a -> IndentParser s u b -> IndentParser s u c

withBlock f a p parses a followed by an indented block of p combining them with f

withBlock' :: Stream s (State SourcePos) z => IndentParser s u a -> IndentParser s u b -> IndentParser s u [b]

Like withBlock, but throws away initial parse result

block :: Stream s (State SourcePos) z => IndentParser s u a -> IndentParser s u [a]

Parses a block of lines at the same indentation level

Indentation Checking

indented :: Stream s (State SourcePos) z => IndentParser s u ()

Parses only when indented past the level of the reference

same :: Stream s (State SourcePos) z => IndentParser s u ()

Parses only on the same line as the reference

sameOrIndented :: Stream s (State SourcePos) z => IndentParser s u ()

Parses only when indented past the level of the reference or on the same line

checkIndent :: Stream s (State SourcePos) z => IndentParser s u ()

Ensures the current indentation level matches that of the reference

withPos :: Stream s (State SourcePos) z => IndentParser s u a -> IndentParser s u a

Parses using the current location for indentation reference

Paired characters

indentBrackets :: Stream s (State SourcePos) z => GenTokenParser s u (State SourcePos) -> IndentParser s u a -> IndentParser s u a

parses with surrounding brackets

indentAngles :: Stream s (State SourcePos) z => GenTokenParser s u (State SourcePos) -> IndentParser s u a -> IndentParser s u a

parses with surrounding angle brackets

indentBraces :: Stream s (State SourcePos) z => GenTokenParser s u (State SourcePos) -> IndentParser s u a -> IndentParser s u a

parses with surrounding braces

indentParens :: Stream s (State SourcePos) z => GenTokenParser s u (State SourcePos) -> IndentParser s u a -> IndentParser s u a

parses with surrounding parentheses

Line Fold Chaining

Any chain using these combinators must used with withPos

(<+/>) :: Stream s (State SourcePos) z => IndentParser s u (a -> b) -> IndentParser s u a -> IndentParser s u b

<+/> is to indentation sensitive parsers what ap is to monads

(<-/>) :: Stream s (State SourcePos) z => IndentParser s u a -> IndentParser s u b -> IndentParser s u a

<-/> is like <+/>, but doesn't apply the function to the parsed value

(<*/>) :: Stream s (State SourcePos) z => IndentParser s u ([a] -> b) -> IndentParser s u a -> IndentParser s u b

Like <+/> but applies the second parser many times

(<?/>) :: Stream s (State SourcePos) z => IndentParser s u (a -> b) -> Optional s u a -> IndentParser s u b

Like <+/> but applies the second parser optionally using the Optional datatype

data Optional s u a

Datatype used to optional parsing

Constructors

Opt a (IndentParser s u a)