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50 lines
1.5 KiB
Haskell
50 lines
1.5 KiB
Haskell
-- |
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-- Module : Text.Megaparsec.ShowToken
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-- Copyright : © 2015 Megaparsec contributors
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-- License : BSD3
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--
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-- Maintainer : Mark Karpov <markkarpov@opmbx.org>
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-- Stability : experimental
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-- Portability : portable
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--
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-- Pretty printing function and instances for use in error messages.
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module Text.Megaparsec.ShowToken (ShowToken (..)) where
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-- | Typeclass 'ShowToken' defines single function 'showToken' that can be
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-- used to “pretty-print” various tokens. By default, all commonly used
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-- instances are defined, but you can add your own, of course.
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class Show a => ShowToken a where
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showToken :: a -> String
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instance ShowToken Char where
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showToken = prettyChar
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-- | @prettyChar ch@ returns user-friendly string representation of given
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-- character @ch@, suitable for using in error messages, for example.
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prettyChar :: Char -> String
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prettyChar '\0' = "null"
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prettyChar '\a' = "bell"
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prettyChar '\b' = "backspace"
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prettyChar '\t' = "tab"
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prettyChar '\n' = "newline"
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prettyChar '\v' = "vertical tab"
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prettyChar '\f' = "form feed"
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prettyChar '\r' = "carriage return"
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prettyChar ' ' = "space"
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prettyChar x = "'" ++ [x] ++ "'"
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instance ShowToken String where
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showToken = prettyString
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-- | @prettyString s@ returns pretty representation of string @s@. This is
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-- used when printing string tokens in error messages.
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prettyString :: String -> String
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prettyString "" = ""
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prettyString [x] = prettyChar x
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prettyString "\r\n" = "crlf newline"
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prettyString xs = "\"" ++ xs ++ "\""
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