I wrote a function and it complies, but I'm not sure if it works the way I intend it to or how to call it in the terminal. Essentially, I want to take a string, like ("age",5),("age",6) and make it into a list of tuples [("age1",5)...]. I am trying to write a function separate the commas and either I am just not sure how to call it in the terminal or I did it wrong.
items :: Parser (String,Integer) -> Parser [(String,Integer)]
items p = do { p <- sepBy strToTup (char ",");
return p }
I'm not sure what you want and I don't know what is Parser.
Starting from such a string:
thestring = "(\"age\",5),(\"age\",6),(\"age\",7)"
I would firstly remove the outer commas with a regular expression method:
import Text.Regex
rgx = mkRegex "\\),\\("
thestring' = subRegex rgx thestring ")("
This gives:
>>> thestring'
"(\"age\",5)(\"age\",6)(\"age\",7)"
Then I would split:
import Data.List.Split
thelist = split (startsWith "(") thestring'
which gives:
>>> thelist
["(\"age\",5)","(\"age\",6)","(\"age\",7)"]
This is what you want, if I correctly understood.
That's probably not the best way. Since all the elements of the final list have form ("age", X) you could extract all numbers (I don't know but it should not be difficult) and then it would be easy to get the final list. Maybe better.
Apologies if this has nothing to do with your question.
Edit
JFF ("just for fun"), another way:
import Data.Char (isDigit)
import Data.List.Split
thestring = "(\"age\",15),(\"age\",6),(\"age\",7)"
ages = (split . dropBlanks . dropDelims . whenElt) (not . isDigit) thestring
map (\age -> "(age," ++ age ++ ")") ages
-- result: ["(age,15)","(age,6)","(age,7)"]
Or rather:
>>> map (\age -> ("age",age)) ages
[("age","15"),("age","6"),("age","7")]
Or if you want integers:
>>> map (\age -> ("age", read age :: Int)) ages
[("age",15),("age",6),("age",7)]
Or if you want age1, age2, ...:
import Data.List.Index
imap (\i age -> ("age" ++ show (i+1), read age :: Int)) ages
-- result: [("age1",15),("age2",6),("age3",7)]
Related
I am stuck at the following parsing problem:
Parse some text string that may contain zero or more elements from a limited character set, up to but not including one of a set of termination characters. Content/no content should be indicated through Maybe. Termination characters may appear in the string in escaped form. Parsing should fail on any inadmissible character.
This is what I came up with (simplified):
import qualified Text.Megaparsec as MP
-- Predicate for admissible characters, not including the control characters.
isAdmissibleChar :: Char -> Bool
...
-- Predicate for control characters that need to be escaped.
isControlChar :: Char -> Bool
...
-- The escape character.
escChar :: Char
...
pComponent :: Parser (Maybe Text)
pComponent = do
t <- MP.many (escaped <|> regular)
if null t then return Nothing else return $ Just (T.pack t)
where
regular = MP.satisfy isAdmissibleChar <|> fail "Inadmissible character"
escaped = do
_ <- MC.char escChar
MP.satisfy isControlChar -- only control characters may be escaped
Say, admissible characters are uppercase ASCII, escape is '\', and control is ':'.
Then, the following parses correctly: ABC\:D:EF to yield ABC:D.
However, parsing ABC&D, where & is inadmissible, does yield ABC whereas I would expect an error message instead.
Two questions:
Why does fail end parsing instead of failing the parser?
Is the above approach sensible to approach the problem, or is there a "proper", canonical way to parse such terminated strings that I am not aware of?
many has to allow its sub-parser to fail once without the whole parse
failing - for example many (char 'A') *> char 'B', while parsing
"AAAB", has to fail to parse the B to know it got to the end of the
As.
You might want manyTill which allows you to recognise the terminator
explicitly. Something like this:
MP.manyTill (escaped <|> regular) (MP.satisfy isControlChar)
"ABC&D" would give an error here assuming '&' isn't accepted by isControlChar.
Or if you want to parse more than one component you might keep your
existing definition of pComponent and use it with sepBy or similar, like:
MP.sepBy pComponent (MP.satisfy isControlChar)
If you also check for end-of-file after this, like:
MP.sepBy pComponent (MP.satisfy isControlChar) <* MP.eof
then "ABC&D" should give an error again, because the '&' will end the first component but will not be accepted as a separator.
What a parser object normally does is to extract from the input stream whatever subset it is supposed to accept. That's the usual rule.
Here, it seems you want the parser to accept strings that are followed by something specific. From your examples, it is either end of file (eof) or character ':'. So you might want to consider look ahead.
Environment and auxiliary functions:
import Data.Void (Void)
import qualified Data.Text as T
import qualified Text.Megaparsec as MP
import qualified Text.Megaparsec.Char as MC
type Parser = MP.Parsec Void T.Text
-- Predicate for admissible characters, not including the control characters.
isAdmissibleChar :: Char -> Bool
isAdmissibleChar ch = elem ch ['A' .. 'Z']
-- Predicate for control characters that need to be escaped.
isControlChar :: Char -> Bool
isControlChar ch = elem ch ":"
-- The escape character:
escChar :: Char
escChar = '\\'
Termination parser, to be used for look ahead:
termination :: Parser ()
termination = MP.eof MP.<|> do
_ <- MP.satisfy isControlChar
return ()
Modified pComponent parser:
pComponent :: Parser (Maybe T.Text)
pComponent = do
txt <- MP.many (escaped MP.<|> regular)
MP.lookAhead termination -- **CHANGE HERE**
if (null txt) then (return Nothing) else (return $ Just (T.pack txt))
where
regular = (MP.satisfy isAdmissibleChar) MP.<|> (fail "Inadmissible character")
escaped = do
_ <- MC.char escChar
MP.satisfy isControlChar -- only control characters may be escaped
Testing utility:
tryParse :: String -> IO ()
tryParse str = do
let res = MP.parse pComponent "(noname)" (T.pack str)
putStrLn $ (show res)
Let's try to rerun your examples:
$ ghci
λ>
λ> :load q67809465.hs
λ>
λ> str1 = "ABC\\:D:EF"
λ> putStrLn str1
ABC\:D:EF
λ>
λ> tryParse str1
Right (Just "ABC:D")
λ>
So that is successful, as desired.
λ>
λ> tryParse "ABC&D"
Left (ParseErrorBundle {bundleErrors = TrivialError 3 (Just (Tokens ('&' :| ""))) (fromList [EndOfInput]) :| [], bundlePosState = PosState {pstateInput = "ABC&D", pstateOffset = 0, pstateSourcePos = SourcePos {sourceName = "(noname)", sourceLine = Pos 1, sourceColumn = Pos 1}, pstateTabWidth = Pos 8, pstateLinePrefix = ""}})
λ>
So that fails, as desired.
Trying our 2 acceptable termination contexts:
λ> tryParse "ABC:&D"
Right (Just "ABC")
λ>
λ>
λ> tryParse "ABCDEF"
Right (Just "ABCDEF")
λ>
fail does not end parsing in general. It just continues with the next alternative. In this case it selects the empty list alternative introduced by the many combinator, so it stops parsing without an error message.
I think the best way to solve your problem is to specify that the input must end in a termination character, that means that it cannot "succeed" halfway like this. You can do that with the notFollowedBy or lookAhead combinators. Here is the relevant part of the megaparsec tutorial.
I'm trying to write a parser for a data Person (data Person). But I have to write it in just one line using <$> and <*> and I was trying a lot, but I'm getting really "overtaxed".
The parser type is as usual:
newtype Parser a = Parser (String -> [(a,String)])
And I have this function:
parse :: Parser a -> String -> Maybe a
that returns the first complete parse.
e.g.
if I have this easy function:
upper :: Parser Char
upper = satisfy isUpper
If I run parse upper "A" I get Just 'A'
I also have a funnier function like this:
name :: Parser String
name = (:) <$> (satisfy isUpper) <*> (many $ satisfy isAlpha)
which, as you can see, accepts all strings that are literal characters and begin with an upper Letter.
so:
*Main> parse name1 "hello"
Nothing
*Main> parse name1 "Hello"
Just "Hello"
Until now is everything fine, the only problem is that I have to do something like that for the class (data, type ?!) Person (data Person)
so, I have this:
data Person = Person String deriving (Eq, Show)
And then, in just one line, I have to write the parser for Person, but the name should satisfy the function name, it means, the name should be just a chain of literal characters, where the first one is upper case.
And it should work so:
> parse parserPerson "Chuck"
Just (Person "Chuck")
> parse parserPerson "chuck"
Nothing
where:
parserPerson :: Parser Person
parserPerson = ???
As you can see, bevor "Chuck" there is Person, so I've to use somehow *> to get it.
And that's it, just a line with <$>, <*> and *> that works that way.
I don't have a clue, and I'm getting crazy with this. Maybe anyone could help me.
EDIT
satisfy :: (Char -> Bool) -> Parser Char -- parse a desired character
satisfy p = Parser check
where
check (c:s) | p c = [(c,s)] -- successful
check _ = [ ] -- no parse
and many (as some) are functions from the Control.Applicative Control.Applicative
As tsorn said, the answer was really easy...
parserPerson :: Parser Person
parserPerson = Person <$> name1
and it works because the Functor Instacnce was defined.
instance Functor Parser where
fmap f (Parser p) = Parser $ \s -> map (\(a,b) -> (f a, b)) $ p s
I'm using the parsec Haskell library.
I want to parse strings of the following kind:
[[v1]][[v2]]
xyz[[v1]][[v2]]
[[v1]]xyz[[v2]]
etc.
I'm interesting to collect only the values v1 and v2, and store these in a data structure.
I tried with the following code:
import Text.ParserCombinators.Parsec
quantifiedVars = sepEndBy var (string "]]")
var = between (string "[[") (string "") (many (noneOf "]]"))
parseSL :: String -> Either ParseError [String]
parseSL input = parse quantifiedVars "(unknown)" input
main = do {
c <- getContents;
case parse quantifiedVars "(stdin)" c of {
Left e -> do { putStrLn "Error parsing input:"; print e; };
Right r -> do{ putStrLn "ok"; mapM_ print r; };
}
}
In this way, if the input is "[[v1]][[v2]]" the program works fine, returning the following output:
"v1"
"v2"
If the input is "xyz[[v1]][[v2]]" the program doesn't work. In particular, I want only what is contained in [[...]], ignoring "xyz".
Also, I want to store the content of [[...]] in a data structure.
How do you solve this problem?
You need to restructure your parser. You are using combinators in very strange locations, and they mess things up.
A var is a varName between "[[" and "]]". So, write that:
var = between (string "[[") (string "]]") varName
A varName should have some kind of format (I don't think that you want to accept "%A¤%&", do you?), so you should make a parser for that; but in case it really can be anything, just do this:
varName = many $ noneOf "]"
Then, a text containing vars, is something with vars separated by non-vars.
varText = someText *> var `sepEndBy` someText
... where someText is anything except a '[':
someText = many $ noneOf "["
Things get more complicated if you want this to be parseable:
bla bla [ bla bla [[somevar]blabla]]
Then you need a better parser for varName and someText:
varName = concat <$> many (try incompleteTerminator <|> many1 (noneOf "]"))
-- Parses e.g. "]a"
incompleteTerminator = (\ a b -> [a, b]) <$> char ']' <*> noneOf "]"
someText = concat <$> many (try incompleteInitiator <|> many1 (noneOf "["))
-- Parses e.g. "[b"
incompleteInitiator = (\ a b -> [a, b]) <$> char '[' <*> noneOf "["
PS. (<*>), (*>) and (<$>) is from Control.Applicative.
I'm using Text.ParserCombinators.Parsec and Text.XHtml to parse an input like this:
- First type A\n
-- First type B\n
- Second type A\n
-- First type B\n
--Second type B\n
And my output should be:
<h1>1 First type A\n</h1>
<h2>1.1 First type B\n</h2>
<h1>2 Second type A\n</h2>
<h2>2.1 First type B\n</h2>
<h2>2.2 Second type B\n</h2>
I have come to this part, but I cannot get any further:
title1= do{
;(count 1 (char '-'))
;s <- many1 anyChar newline
;return (h1 << s)
}
title2= do{
;(count 2 (char '--'))
;s <- many1 anyChar newline
;return (h1 << s)
}
text=do {
;many (choice [try(title1),try(title2)])
}
main :: IO ()
main = do t putStr "Error: " >> print err
Right x -> putStrLn $ prettyHtml x
This is ok, but it does not include the numbering.
Any ideas?
Thanks!
You probably want to use GenParser with a state containing the current section numbers as a list in reverse order, so section 1.2.3 will be represented as [3,2,1], and maybe the length of the list to avoid repeatedly counting it. Something like
data SectionState = SectionState {nums :: [Int], depth :: Int}
Then make your parser actions return type be "GenParser Char SectionState a". You can access the current state in your parser actions using "getState" and "setState". When you get a series of "-" at the start of a line count them and compare it with "depth" in the state, manipulate the "nums" list appropriately, and then emit "nums" in reverse order (I suggest keeping nums in reverse order because most of the time you want to access the least significant item, so putting it at the head of the list is both easier and more efficient).
See Text.ParserCombinators.Parsec.Prim for details of GenParser. The more usual Parser type is just "type Parser a = GenParser Char () a" You probably want to say
type MyParser a = GenParser Char SectionState a
somewhere near the start of your code.
I am writing a custom read function for one of the data types in my module. For eg, when I do read "(1 + 1)" :: Data, I want it to return Plus 1 1. My data declaration is data Data = Plus Int Int. Thanks
This sounds like something better suited to a parser; Parsec is a powerful Haskell parser combinator library, which I would recommend.
I'd like to second the notion of using a parser. However, if you absolutely have to use a pattern-matching, go like this:
import Data.List
data Expr = Plus Int Int | Minus Int Int deriving Show
test = [ myRead "(1 + 1)", myRead "(2-1)" ]
myRead = match . lexer
where
match ["(",a,"+",b,")"] = Plus (read a) (read b)
match ["(",a,"-",b,")"] = Minus (read a) (read b)
match garbage = error $ "Cannot parse " ++ show garbage
lexer = unfoldr next_lexeme
where
next_lexeme "" = Nothing
next_lexeme str = Just $ head $ lex str
You could use GHC's ReadP.