Re: [Haskell-cafe] parsing exercise
On Sun, Jan 23, 2011 at 4:31 PM, Chung-chieh Shan
wrote:
> Maybe Text.Show.Pretty.parseValue in the pretty-show package can help?
>
That's what I was looking for, thanks!
On Sun, Jan 23, 2011 at 5:23 PM, Stephen Tetley
wrote:
> I don't think you can do this "simply" as you think you would always
> have to build a parse tree.
Isn't it enough to maintain a stack of open parens, brackets, char- and
string-terminators and escape chars? Below is my attempt at solving the
problem without an expression parser.
In practice, if you follow the skeleton syntax tree style you might
> find "not caring" about the details of syntax is almost as much work
> as caring about them. I've tried a couple of times to make a skeleton
> parser that does paren nesting and little else, but always given up
> and just used a proper parser as the skeleton parser never seemed
> robust.
>
Indeed I doubt that the implementation below is robust and it's too tricky
to be easily maintainable. I include it for reference. Let me know if you
spot an obvious mistake..
Sebastian
splitTLC :: String -> [String]
splitTLC = parse ""
type Stack = String
parse :: Stack -> String -> [String]
parse _ "" = []
parse st (c:cs) = next c st $ parse (updStack c st) cs
next :: Char -> Stack -> [String] -> [String]
next c []xs = if c==',' then [] : xs else c <: xs
next c (_:_) xs = c <: xs
infixr 0 <:
(<:) :: Char -> [String] -> [String]
c <: [] = [[c]]
c <: (x:xs) = (c:x):xs
updStack :: Char -> Stack -> Stack
updStack char stack =
case (char,stack) of
-- char is an escaped character
(_ ,'\\':xs) -> xs -- the next character is not
-- char is the escape character
('\\', xs) -> '\\':xs -- push it on the stack
-- char is the string terminator
('"' , '"':xs) -> xs -- closes current string literal
('"' , ''':xs) -> ''':xs -- ignored inside character
('"' , xs) -> '"':xs -- opens a new string
-- char is the character terminator
(''' , ''':xs) -> xs -- closes current character literal
(''' , '"':xs) -> '"':xs -- ignored inside string
(''' , xs) -> ''':xs -- opens a new character
-- parens and brackets
(_ , '"':xs) -> '"':xs -- are ignored inside strings
(_ , ''':xs) -> ''':xs -- and characters
('(' , xs) -> '(':xs -- new opening paren
(')' , '(':xs) -> xs -- closing paren
('[' , xs) -> '[':xs -- opening bracket
(']' , '[':xs) -> xs -- closing bracket
-- other character don't modify the stack (ignoring record syntax)
(_ , xs) -> xs
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Re: [Haskell-cafe] parsing exercise
I don't think you can do this "simply" as you think you would always have to build a parse tree. If the input is valid Haskell you could follow Chung-chieh Shan's suggestion, otherwise you could parse to a "skeleton syntax tree" - look for work by Jonathan Bacharach on Dylan macros and "Java Syntax Extension". In practice, if you follow the skeleton syntax tree style you might find "not caring" about the details of syntax is almost as much work as caring about them. I've tried a couple of times to make a skeleton parser that does paren nesting and little else, but always given up and just used a proper parser as the skeleton parser never seemed robust. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] parsing exercise
Sebastian Fischer wrote in article in gmane.comp.lang.haskell.cafe: > I expect writing this function to be quite tedious (ignore commas in parens, > ignore parens in strings, quotation, ...) and would prefer to copy code from > some parsing library. Do you have an idea what I could use? Or how to solve > it from scratch in a few lines? Maybe Text.Show.Pretty.parseValue in the pretty-show package can help? -- Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig Insert wit here. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] parsing exercise
You might want to check out parsec, and the chapter related to it in RWH.
http://book.realworldhaskell.org/read/using-parsec.html
On Sun, Jan 23, 2011 at 12:09 AM, Sebastian Fischer wrote:
> Hello,
>
> I need a function and wonder whether I can copy some existing code so I
> don't have to write it myself.
>
> It should split a string into a list of strings:
>
> splitAtTopLevelCommas :: String -> [String]
>
> I need something similar to `splitOn ","` from the Text package with the
> property
>
> intercalate "," . splitAtTopLevelCommas = id
>
> But it should split the string only on a few commas, not all. You can think
> of the result list as representations of Haskell values, for example
>
> splitAtTopLevelCommas "True,(1,(2,[3,4])),Just ('a',\")\")"
>
> should yield
>
> ["True", "(1,(2,[3,4]))", "Just ('a',\")\")"]
>
> I expect writing this function to be quite tedious (ignore commas in
> parens, ignore parens in strings, quotation, ...) and would prefer to copy
> code from some parsing library. Do you have an idea what I could use? Or how
> to solve it from scratch in a few lines?
>
> Sebastian
>
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>
>
--
Alex R
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[Haskell-cafe] parsing exercise
Hello,
I need a function and wonder whether I can copy some existing code so I
don't have to write it myself.
It should split a string into a list of strings:
splitAtTopLevelCommas :: String -> [String]
I need something similar to `splitOn ","` from the Text package with the
property
intercalate "," . splitAtTopLevelCommas = id
But it should split the string only on a few commas, not all. You can think
of the result list as representations of Haskell values, for example
splitAtTopLevelCommas "True,(1,(2,[3,4])),Just ('a',\")\")"
should yield
["True", "(1,(2,[3,4]))", "Just ('a',\")\")"]
I expect writing this function to be quite tedious (ignore commas in parens,
ignore parens in strings, quotation, ...) and would prefer to copy code from
some parsing library. Do you have an idea what I could use? Or how to solve
it from scratch in a few lines?
Sebastian
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