[Haskell-cafe] Parsec monad transformer with IO?
Hello!
Nice, Parsec 3 comes with a monad transformer [1]. So I thought I could
use IO as inner monad, and perform IO operations during parsing.
But I failed. Monad transformers still bend my mind. My problem: I
don't see a function to actually lift the IO operation into the
ParsecT. It should be something like
lift :: IO a - ParsecT s u IO a
The following is a toy example, I just could not make something
smaller: Let's parse command line arguments (tokens are Strings), and
execute them while parsing.
import Text.Parsec.Prim
import Text.Parsec.Pos
import Text.Parsec
import System.Environment ( getArgs )
Command line interface parser (Clip) type: Inner monad IO, user state
u, tokens are Strings, returns something typed a.
type Clip u a = ParsecT [String] u IO a
Source code position for command line arguments: The line is always 1,
column n represents the n-th command line argument.
nextPos p _ _ = incSourceColumn p 1
Two primitive parsers, one for flags (with a dash) and one for other
arguments:
clipFlag x accepts the command line flag -x, and returns x.
clipFlag :: String - Clip u String
clipFlag x
= tokenPrim
id
nextPos
(\y - if '-':x == y then Just x else Nothing)
clipValue accepts any command line argument that does not tart with a
dash '-'.
clipValue :: Clip u String
clipValue
= tokenPrim id nextPos test
where
test ('-':_) = Nothing
test other = Just other
Now the test program:
Load files given on the command line, and sum up their word count,
until -p is given. -p prints the current word count and sets the
counter to zero. Further files may be processed then. At the end, show
the sum of all word counts.
Example: foo has 12 words, bar has 34 words:
main foo -p bar -p foo bar -p
Counted 12 words, reset counter.
Counted 34 words, reset counter.
Counted 46 words, reset counter.
Grand total: 92
type CurrentCount = Int -- the user state used with Clip/ParsecT.
root implements the command line grammar (filename+ -p)* and
returns the sum of all word counts.
root :: Clip CurrentCount Int
root
= do ns - many (many1 loadFile printSize)
eof
return $ sum ns
Interprets each non-flag on the command line as filename, loads it,
counts its words, and adds the count to the state.
loadFile :: Clip CurrentCount ()
loadFile
= do -- expect a filename
filename - clipValue
-- load the file: IO
content - lift $ readFile filename
-- add wordcount to state
modifyState ((+) (length $ words content))
If -p shows up on the command line, print accumulated count, reset
counter to cero and return count for grand-total calculation.
printSize :: Clip CurrentCount Int
printSize
= do -- expect flag -p
clipFlag p
-- print current word count: IO
n - getState
lift . putStrLn $ Counted ++show n++ words, reset counter.
-- reset user state to zero, return word count for grand total
putState 0
return n
main just runs the root parser on the command line arguments and
checks the result.
main
= do result - getArgs = runParserT root 0 command line
case result of
Left err - error $ show err
Right n - putStrLn $ Grand total: ++show n
So where is the lift function? Does it exist? Here, I need your help.
lift :: IO a - ParsecT s u IO a
lift = undefined
Any comments are appreciated.
Thank you!
Stefan
[1]
http://hackage.haskell.org/packages/archive/parsec/3.0.0/doc/html/Text-Parsec-Prim.html#t:ParsecT
--
Stefan Klinger o/klettern
/\/ bis zum
send plaintext only - max size 32kB - no spam \ Abfallen
http://stefan-klinger.de
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Re: [Haskell-cafe] Parsec monad transformer with IO?
Stefan Klinger all-li...@stefan-klinger.de writes: Hello! Nice, Parsec 3 comes with a monad transformer [1]. So I thought I could use IO as inner monad, and perform IO operations during parsing. But I failed. Monad transformers still bend my mind. My problem: I don't see a function to actually lift the IO operation into the ParsecT. It should be something like lift :: IO a - ParsecT s u IO a ParsecT has a MonadIO instance: class Monad m = MonadIO m where liftIO :: IO a - m a G -- Gregory Collins g...@gregorycollins.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parsec monad transformer with IO?
On Thu, Mar 18, 2010 at 10:37 AM, Stefan Klinger
all-li...@stefan-klinger.de wrote:
Hello!
Nice, Parsec 3 comes with a monad transformer [1]. So I thought I could
use IO as inner monad, and perform IO operations during parsing.
But I failed. Monad transformers still bend my mind. My problem: I
don't see a function to actually lift the IO operation into the
ParsecT. It should be something like
lift :: IO a - ParsecT s u IO a
That operation, with that name, and (a generalization of) that type,
is *the* method of the MonadTrans class. Essentially the presence of
that operation is the definition of what it means to be a monad
transformer.
The following is a toy example, I just could not make something
smaller: Let's parse command line arguments (tokens are Strings), and
execute them while parsing.
import Text.Parsec.Prim
import Text.Parsec.Pos
import Text.Parsec
import System.Environment ( getArgs )
Command line interface parser (Clip) type: Inner monad IO, user state
u, tokens are Strings, returns something typed a.
type Clip u a = ParsecT [String] u IO a
Source code position for command line arguments: The line is always 1,
column n represents the n-th command line argument.
nextPos p _ _ = incSourceColumn p 1
Two primitive parsers, one for flags (with a dash) and one for other
arguments:
clipFlag x accepts the command line flag -x, and returns x.
clipFlag :: String - Clip u String
clipFlag x
= tokenPrim
id
nextPos
(\y - if '-':x == y then Just x else Nothing)
clipValue accepts any command line argument that does not tart with a
dash '-'.
clipValue :: Clip u String
clipValue
= tokenPrim id nextPos test
where
test ('-':_) = Nothing
test other = Just other
Now the test program:
Load files given on the command line, and sum up their word count,
until -p is given. -p prints the current word count and sets the
counter to zero. Further files may be processed then. At the end, show
the sum of all word counts.
Example: foo has 12 words, bar has 34 words:
main foo -p bar -p foo bar -p
Counted 12 words, reset counter.
Counted 34 words, reset counter.
Counted 46 words, reset counter.
Grand total: 92
type CurrentCount = Int -- the user state used with Clip/ParsecT.
root implements the command line grammar (filename+ -p)* and
returns the sum of all word counts.
root :: Clip CurrentCount Int
root
= do ns - many (many1 loadFile printSize)
eof
return $ sum ns
Interprets each non-flag on the command line as filename, loads it,
counts its words, and adds the count to the state.
loadFile :: Clip CurrentCount ()
loadFile
= do -- expect a filename
filename - clipValue
-- load the file: IO
content - lift $ readFile filename
-- add wordcount to state
modifyState ((+) (length $ words content))
If -p shows up on the command line, print accumulated count, reset
counter to cero and return count for grand-total calculation.
printSize :: Clip CurrentCount Int
printSize
= do -- expect flag -p
clipFlag p
-- print current word count: IO
n - getState
lift . putStrLn $ Counted ++show n++ words, reset counter.
-- reset user state to zero, return word count for grand total
putState 0
return n
main just runs the root parser on the command line arguments and
checks the result.
main
= do result - getArgs = runParserT root 0 command line
case result of
Left err - error $ show err
Right n - putStrLn $ Grand total: ++show n
So where is the lift function? Does it exist? Here, I need your help.
lift :: IO a - ParsecT s u IO a
lift = undefined
Any comments are appreciated.
Thank you!
Stefan
[1]
http://hackage.haskell.org/packages/archive/parsec/3.0.0/doc/html/Text-Parsec-Prim.html#t:ParsecT
--
Stefan Klinger o/klettern
/\/ bis zum
send plaintext only - max size 32kB - no spam \ Abfallen
http://stefan-klinger.de
___
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Re: [Haskell-cafe] Parsec monad transformer with IO?
On 18 March 2010, Gregory Collins wrote with possible deletions: ParsecT has a MonadIO instance: class Monad m = MonadIO m where liftIO :: IO a - m a Thank you! I didn't see this. Great! Kind regards, Stefan -- Stefan Klinger o/klettern /\/ bis zum send plaintext only - max size 32kB - no spam \ Abfallen http://stefan-klinger.de ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parsec monad transformer with IO?
Hoogle is a great tool for finding haskell functions: http://www.haskell.org/hoogle/ You can punch in the type of a function you want and it will give you a list of functions that might do what you need. Generalizing the types a bit usually helps. Searching for either m a - n m a or IO a - m a would give you 'lift' and 'liftIO' as one of the top results. - Job On Thu, Mar 18, 2010 at 1:58 PM, Stefan Klinger all-li...@stefan-klinger.de wrote: On 18 March 2010, Gregory Collins wrote with possible deletions: ParsecT has a MonadIO instance: class Monad m = MonadIO m where liftIO :: IO a - m a Thank you! I didn't see this. Great! Kind regards, Stefan -- Stefan Klinger o/klettern /\/ bis zum send plaintext only - max size 32kB - no spam \ Abfallen http://stefan-klinger.de ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parsec monad transformer with IO?
Job Vranish wrote: Hoogle is a great tool for finding haskell functions: http://www.haskell.org/hoogle/ You can punch in the type of a function you want and it will give you a list of functions that might do what you need. Generalizing the types a bit usually helps. Searching for either m a - n m a or IO a - m a would give you 'lift' and 'liftIO' as one of the top results. Is there a tool anywhere which can figure out how to construct a function with a specific type signature? Hoogle works if the thing you seek is a single function, but not so much if you need to throw several functions together. (For example, the signature x - [x - y] - [y] can be implemented by \ x - map ($ x), but this is initially non-obvious.) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parsec monad transformer with IO?
On 18 March 2010 21:34, Andrew Coppin andrewcop...@btinternet.com wrote: Is there a tool anywhere which can figure out how to construct a function with a specific type signature? Hoogle works if the thing you seek is a single function, but not so much if you need to throw several functions together. Hi Andrew There is Lennart Augustsson's Djinn Oleg Kiselyov's 'de-typechecker' http://hackage.haskell.org/package/djinn Reversing Haskell typechecker: converting from undefined to defined http://okmij.org/ftp/Haskell/types.html Clean has something similar to generate test functions for GAST - Clean's equivalent to QuickCheck. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
