- Haskell-Cafe - Haskell.org

Parsing integers [was: Re: [Haskell-cafe] help]
by Tim Chevalier 10 Dec '07

10 Dec '07

On 12/9/07, Ryan Bloor <ryanbloor(a)hotmail.com> wrote: > > hi > > I have a function parseInt... which needs an error guard for when the input > is not an Int. > > parseInt :: Parser > parseInt [] = [] > parseInt xs = let (digits, rest) = span isDigit (removeSpace xs) > in [(EInt (read digits), removeSpace rest)] > > Also... I have a function that does this... parseBinaryOp "+" "(5 + 2) if" > gives...[(Int 5, Int 2, "if")] > so, op is '+' or "&&". I am unsure of how to begin... > > parseBinaryOp :: String -> String -> [(Expr, Expr, String)] > parseBinaryOp op str > Check out: http://www.haskell.org/haskellwiki/Homework_help If this isn't homework, try posting again and phrasing your question in the form of a question; saying "I am unsure of how to begin" doesn't help us help you think about the problem. Cheers, Tim -- Tim Chevalier * catamorphism.org * Often in error, never in doubt "[Teaching children to read] will make America what we want it to be, a literate country and a hopefuller country." -- George W. Bush

2 2

Point and link
by Andrew Coppin 09 Dec '07

09 Dec '07

Hi guys. Here's a fairly basic question. I have several ideas for programs that I'd like to write. They all involve placing "units" of some kind, and then drawing "connections" between those units. How feasible is it to program such a thing in Haskell? Where would you start? (Gtk2hs is the only graphics API I'm familiar with using at present.)

8 13

Fwd: Parsing integers [was: Re: [Haskell-cafe] help]
by Tim Chevalier 09 Dec '07

09 Dec '07

[reply forwarded to list] ---------- Forwarded message ---------- From: Ryan Bloor <ryanbloor(a)hotmail.com> hi sorry for vagueness. I am writing a basic Parser from scratch. So far I have functions; # removeSpaces # match - which checks if a string is a substring of another # orParser which combines two parser's abilities # Basic pasrers like... parseInt, parseTrue, parseFalse, parseBool using the orParser on True and False. What I want to do now is have a parseBinaryOp that recognises: parseBinaryOp "+" "(5 + 2) if" >>>gives>> [(EInt 5, EInt 2, "if")] So I think that I have to split the initial string into four parts. "+" becomes op '(' becomes tokenF ')' becomes tokenB "5" becomes e1 "2" becomes e2 parseBinaryOp :: String -> String -> [(Expr, Expr, String)] parseBinaryOp op str = let (tokenF,e1,op,e2,tokenB) = I am not sure how to go about separating the string for how I need it using my other functiuons. Ryan ________________________________ > Date: Sun, 9 Dec 2007 13:07:23 -0800 > From: catamorphism(a)gmail.com > To: ryanbloor(a)hotmail.com > Subject: Parsing integers [was: Re: [Haskell-cafe] help] > CC: haskell-cafe(a)haskell.org > > On 12/9/07, Ryan Bloor <ryanbloor(a)hotmail.com> wrote: > > > > hi > > > > I have a function parseInt... which needs an error guard for when the input > > is not an Int. > > > > parseInt :: Parser > > parseInt [] = [] > > parseInt xs = let (digits, rest) = span isDigit (removeSpace xs) > > in [(EInt (read digits), removeSpace rest)] > > > > Also... I have a function that does this... parseBinaryOp "+" "(5 + 2) if" > > gives...[(Int 5, Int 2, "if")] > > so, op is '+' or "&&". I am unsure of how to begin... > > > > parseBinaryOp :: String -> String -> [(Expr, Expr, String)] > > parseBinaryOp op str > > > > Check out: > http://www.haskell.org/haskellwiki/Homework_help > > If this isn't homework, try posting again and phrasing your question > in the form of a question; saying "I am unsure of how to begin" > doesn't help us help you think about the problem. > > Cheers, > Tim > > -- > Tim Chevalier * catamorphism.org * Often in error, never in doubt > "[Teaching children to read] will make America what we want it to be, > a literate country and a hopefuller country." -- George W. Bush ________________________________ Are you the Quizmaster? Play BrainBattle with a friend now!

1 0

help
by Ryan Bloor 09 Dec '07

09 Dec '07

hi I have a function parseInt... which needs an error guard for when the input is not an Int. parseInt :: ParserparseInt [] = []parseInt xs = let (digits, rest) = span isDigit (removeSpace xs) in [(EInt (read digits), removeSpace rest)] Also... I have a function that does this... parseBinaryOp "+" "(5 + 2) if" gives...[(Int 5, Int 2, "if")]so, op is '+' or "&&". I am unsure of how to begin... parseBinaryOp :: String -> String -> [(Expr, Expr, String)]parseBinaryOp op str Thankyou Ryan _________________________________________________________________ Get free emoticon packs and customisation from Windows Live. http://www.pimpmylive.co.uk

1 0

ARM back end?
by Greg Fitzgerald 09 Dec '07

09 Dec '07

Anybody know of an ARM back end for any of the Haskell compilers? Thanks, Greg

8 7

ST Monad - what's wrong?
by Nicu Ionita 09 Dec '07

09 Dec '07

Hi, I'm trying to use the ST monad in order to turn an almost pure function into a pure one: reading a precalculated list of primes into a prime set. But the following code brings an error: > primes :: Set Integer > primes = runST $ getPrimes "primes10h7.txt" > getPrimes :: String -> (forall s. ST s (Set Integer)) > getPrimes file = > do cont <- unsafeIOToST (readFile file) > let set = fromList $ map read $ lines cont > return set And here is the error: Couldn't match expected type `forall s. ST s a' against inferred type `ST s (Set Integer)' In the second argument of `($)', namely `getPrimes "primes10h7.txt"' In the expression: runST $ (getPrimes "primes10h7.txt") In the definition of `primes': primes = runST $ (getPrimes "primes10h7.txt") Compiled with GHC 6.6.1 with extensions activated. Nicu

4 4

AW: ST Monad - what's wrong?
by Nicu Ionita 09 Dec '07

09 Dec '07

Ok, solved, the use of ($) was the problem. I changed it > primes :: Set Integer > primes = runST (getPrimes "primes10h7.txt") and it compiles (and works too). Nicu > -----Ursprüngliche Nachricht----- > Von: Nicu Ionita [mailto:nionita@lycos.de] > Gesendet: Sonntag, 9. Dezember 2007 12:12 > An: 'haskell-cafe(a)haskell.org' > Betreff: ST Monad - what's wrong? > > > Hi, > > I'm trying to use the ST monad in order to turn an almost > pure function into a pure one: reading a precalculated list > of primes into a prime set. But the following code brings an error: > > > primes :: Set Integer > > primes = runST $ getPrimes "primes10h7.txt" > > > getPrimes :: String -> (forall s. ST s (Set Integer)) > getPrimes file = > > do cont <- unsafeIOToST (readFile file) > > let set = fromList $ map read $ lines cont > > return set > > And here is the error: > > Couldn't match expected type `forall s. ST s a' > against inferred type `ST s (Set Integer)' > In the second argument of `($)', namely > `getPrimes "primes10h7.txt"' > In the expression: runST $ (getPrimes "primes10h7.txt") > In the definition of `primes': > primes = runST $ (getPrimes "primes10h7.txt") > > Compiled with GHC 6.6.1 with extensions activated. > > Nicu >

1 0

Re: [Haskell-cafe] do... error
by Felipe Lessa 09 Dec '07

09 Dec '07

On Dec 9, 2007 1:01 AM, Ryan Bloor <ryanbloor(a)hotmail.com> wrote: > But what is the right way to indent...? It is so annoying, why does it > matter so much! :( You may read http://en.wikibooks.org/wiki/Haskell/Indentation which tries to explain in a very simple language. -- Felipe.

3 3

do... error
by Ryan Bloor 09 Dec '07

09 Dec '07

hi test :: Parser (Char,Char) test = do x <- item item y <- item return (x,y) How come this brings an error saying that after do {....} it must end with an expression. Ryan _________________________________________________________________ Get free emoticon packs and customisation from Windows Live. http://www.pimpmylive.co.uk

3 3

Re: [Haskell-cafe] Type error in final generator
by Loganathan Lingappan 09 Dec '07

09 Dec '07

Thanks Bryan and Derek. This works! Logo ----- Original Message ---- From: Derek Elkins <derek.a.elkins(a)gmail.com> To: Bryan O'Sullivan <bos(a)serpentine.com> Cc: Loganathan Lingappan <loganathan.lingappan(a)yahoo.com>; haskell-cafe(a)haskell.org Sent: Saturday, December 8, 2007 4:53:54 PM Subject: Re: [Haskell-cafe] Type error in final generator On Sat, 2007-12-08 at 16:39 -0800, Bryan O'Sullivan wrote: > Loganathan Lingappan wrote: > > > main = do > > hSetBuffering stdin LineBuffering > > numList <- processInputs > > foldr (+) 0 numList > > The type of main is understood to be IO (), so it can't return anything. > You could work around this by rewriting the last line above as follows: > > print (foldr (+) 0 numList) > > This prints the number, which is presumably what you want, and print has > type IO (), so it works out nicely here. Nitpicking: Actually, as the error message says, the type of main is IO a so it can 'return' -anything- and that will be discarded as the type makes clear. The issue is that foldr (+) 0 numList :: Integer and that's not IO a for any a. Either way, Bryan's suggestion is probably what you want.

1 0