parsec

问题 Traditionally, arithmetic operators are considered to be binary (left or right associative), thus most tools are dealing only with binary operators. Is there an easy way to parse arithmetic operators with Parsec, which can have an arbitrary number of arguments? For example, the following expression should be parsed into the tree (a + b) + c + d * e + f 回答1: Yes! The key is to first solve a simpler problem, which is to model + and * as tree nodes with only two children. To add four things, we

问题 Traditionally, arithmetic operators are considered to be binary (left or right associative), thus most tools are dealing only with binary operators. Is there an easy way to parse arithmetic operators with Parsec, which can have an arbitrary number of arguments? For example, the following expression should be parsed into the tree (a + b) + c + d * e + f 回答1: Yes! The key is to first solve a simpler problem, which is to model + and * as tree nodes with only two children. To add four things, we

问题 I am trying to parse some comma separated string which may or may not contain a string with image dimensions. For example "hello world, 300x300, good bye world" . I've written the following little program: import Text.Parsec import qualified Text.Parsec.Text as PS parseTestString :: Text -> [Maybe (Int, Int)] parseTestString s = case parse dimensStringParser "" s of Left _ -> [Nothing] Right dimens -> dimens dimensStringParser :: PS.Parser [Maybe (Int, Int)] dimensStringParser = (optionMaybe

问题 I am trying to parse some comma separated string which may or may not contain a string with image dimensions. For example "hello world, 300x300, good bye world" . I've written the following little program: import Text.Parsec import qualified Text.Parsec.Text as PS parseTestString :: Text -> [Maybe (Int, Int)] parseTestString s = case parse dimensStringParser "" s of Left _ -> [Nothing] Right dimens -> dimens dimensStringParser :: PS.Parser [Maybe (Int, Int)] dimensStringParser = (optionMaybe

问题 I am using the Bill Casarin post on how to parse delimited files with fparsec, I am dumbing the logic down to get an understanding of how the code works. I am parsing a multi row delimited document into Cell list list structure (for now) where a Cell is a string or a float. I am a complete newbie on this. I am having issues parsing the floats - in a typical case (a cell delimitted by tabs, containing a numeric) it works. However when a cell happens to be a string that starts with a number -

问题 I am using the Bill Casarin post on how to parse delimited files with fparsec, I am dumbing the logic down to get an understanding of how the code works. I am parsing a multi row delimited document into Cell list list structure (for now) where a Cell is a string or a float. I am a complete newbie on this. I am having issues parsing the floats - in a typical case (a cell delimitted by tabs, containing a numeric) it works. However when a cell happens to be a string that starts with a number -

问题 using a cabal file looking like this (the relevant library part): build-depends: base >=4.8 && < 4.9, filepath >=1.4 && <1.5, time >=1.5 && <1.6, bytestring >=0.10 && <0.11, unix >=2.7 && <2.8, cryptohash >=0.11 && <0.12, process >=1.2 && <1.3, transformers >= 0.4 && < 0.5, text >= 1.2 && <= 1.3, base16-bytestring >= 0.1.1 && < 1.1.2, utf8-string >= 1 && < 1.1, directory >=1.2 && <1.3, regex-base >= 0.9 && < 1.0, regex-pcre >= 0.94 && < 0.95, regex-base >= 0.93 && < 0.94, direct-sqlite >=2.3

问题 I have this parsec parser : a = optionMaybe $ do {try $ spaceornull *> string "hello";string "No"} Where spaceornull is ((:[]) <$> try space) <|> string "" When I test a with input " " I get : Left (line 1, column 2): unexpected end of input expecting "hello" I don't understand this, spaceornull *> string "hello" should fail because there is no "hello", then with try parsec backtracks and now there is no consumed input but try fails anyway so the parser passed to optionMaybe (the one inside

问题 I have this parsec parser : a = optionMaybe $ do {try $ spaceornull *> string "hello";string "No"} Where spaceornull is ((:[]) <$> try space) <|> string "" When I test a with input " " I get : Left (line 1, column 2): unexpected end of input expecting "hello" I don't understand this, spaceornull *> string "hello" should fail because there is no "hello", then with try parsec backtracks and now there is no consumed input but try fails anyway so the parser passed to optionMaybe (the one inside

问题 I'm trying to write the following parser using parsec: manyLength :: forall s u m a. Monad m => ParsecT s u m a -> ParsecT s u m Int manyLength p = go 0 where go :: Int -> ParsecT s u m Int go !i = (p *> go (i + 1)) <|> pure i This is like the many function, but instead of returning [a] , it returns the number of times Parser a succeeds. This works, but I can't seem to make it run in constant heap space. This makes sense, since the recursive call to go is not in the tail-call position. If