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|
module Data.Primitive.Internal.Read
( Tag(..)
, lexTag
) where
import Data.Char (isDigit)
import Text.ParserCombinators.ReadP
data Tag = FromListTag | FromListNTag
-- Why don't we just use lexP? The general problem with lexP is that
-- it doesn't always fail as fast as we might like. It will
-- happily read to the end of an absurdly long lexeme (e.g., a 200MB string
-- literal) before returning, at which point we'll immediately discard
-- the result because it's not an identifier. Doing the job ourselves, we
-- can see very quickly when we've run into a problem. We should also get
-- a slight efficiency boost by going through the string just once.
lexTag :: ReadP Tag
lexTag = do
_ <- string "fromList"
s <- look
case s of
'N':c:_
| isDigit c
-> fail "" -- We have fromListN3 or similar
| otherwise -> FromListNTag <$ get -- Skip the 'N'
_ -> return FromListTag
|
