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# Replacing the Read Class
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## Tickets
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<table><tr><th>[\#61](https://gitlab.haskell.org//haskell/prime/issues/61)</th>
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<td>replace the Read class</td></tr></table>
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## Problems with the Haskell 98 class
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- the backtracking parser is very inefficient
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- writing instances is painful, compared to a monadic interface
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| ... | ... | @@ -15,44 +12,19 @@ |
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## Proposed replacements
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- parsers based on [ ReadP](http://www.haskell.org/ghc/dist/current/docs/libraries/base/Text-ParserCombinators-ReadP.html), from [ Parallel Parsing Processes](http://www.md.chalmers.se/~koen/Papers/parsing-pearl.ps) by Koen Classen, JFP 2004.
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- abstract type with monadic interface, invoked with
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- parsers based on [ ReadP](http://www.haskell.org/ghc/docs/latest/html/libraries/base/Text-ParserCombinators-ReadP.html), from [ Parallel Parsing Processes](http://www.md.chalmers.se/~koen/Papers/parsing-pearl.ps) by Koen Classen, JFP 2004.
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```wiki
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readP_to_S :: ReadP a -> String -> [(a, String)]
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```
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(The internal representation is more complex.)
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- choice operator supports full nondeterminism by parsing alternatives in parallel.
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(There is also a committed choice operator.)
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- monadic interface
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- supports nondeterminism without needing explicit committed choice
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- mostly deterministic parsers are much more efficient than `ReadS`
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- returns no information on failure, but could be altered to do so, as it knows the position at which all alternatives are exhausted.
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- no error reporting
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- requires [PolymorphicComponents](polymorphic-components)
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GHC has used a new `Read` class based on these parsers since 5.04 (July 2002).
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- Malcolm's experimental [ Poly TextParser](http://nhc98.blogspot.com/#113301726218243351) in the development version of [ HaXml](http://www.cs.york.ac.uk/fp/HaXml-1.14/).
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- monadic interface, invoked with
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```wiki
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runParser :: TextParser a -> String -> (Either String a, String)
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```
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(The internal representation is more general, parameterised on token type.)
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- returns only the first successful alternative, but with internal backtracking,
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and an explicit commit operator to prevent backtracking where necessary
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- on failure, returns an error string and the unparsed input
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- failure messages can be defined/combined/nested with convenient combinators
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- Could also use something based on Daan's [ Parsec](http://www.cs.uu.nl/people/daan/parsec.html) combinators
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- monadic interface, invoked with (simplifying slightly)
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- Malcolm's experimental Poly parser in the CVS version of HaXml?.
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```wiki
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parse :: Parser a -> SourceName -> String -> Either ParseError a
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```
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- mostly deterministic, with explicit 'try' operator for non-determinism
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- on failure, reports an error message including the position at which the error occured.
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- already widely used for other parsing tasks |
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- monadic interface
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- deterministic parser with committed choice
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- returns an error string on failure |
