Re: [Haskell-cafe] Re: request for code review
On Tuesday 14 March 2006 20:58, you wrote: On 3/14/06, Benjamin Franksen [EMAIL PROTECTED] wrote: On Tuesday 14 March 2006 14:46, Pete Chown wrote: Shannon -jj Behrens wrote: Arrows looks like a replacement for monads. Are you saying I should drop my use of the State monad? If so, why? I like the readability of the do syntax. Okay, now it's my turn to ask a question. :-) I've read about arrows, and while I think I see what they do, I'm not sure why they are seen as so special that they even get new syntax. This question of Shannon's is exactly the point I struggle with. I can see that the arrow operators might be useful with functions, but are they useful for other things too? Yes, http://www.haskell.org/arrows/biblio.html lists a number of papers describing non-trivial applications of Arrows, that is, Arrows other than (-). I found the exposition in http://www.haskell.org/yale/papers/oxford02/ to be quite readable. For example, as monads are one kind of arrow, I thought I would make some of the I/O functions into arrows and see what happened. The result was pretty much the same as using the monad, except slightly less convenient. You can write monadic code without ever using the syntax sugar, and get along. However, do-notation is convenient. OTOH, I am told that programming with Arrows is really quite inconvenient w/o the syntax sugar. Well, forgive me for my newbie-ness: o How important is it that I switch from using the State monad to using arrows? o How important is it that I switch from using | or $ to using arrows? (It seems that using arrows just to replace | or $ is like using a sledge hammer to drive a thumb tack.) o How much will this increase the conceptual complexity of my program--i.e. how much time am I going to have to spend explaining it in my article? o How much will this improve the readability or decrease the amount of code in my program? Thanks! -jj ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: request for code review
On Tuesday 14 March 2006 20:58, you wrote: On 3/14/06, Benjamin Franksen [EMAIL PROTECTED] wrote: On Tuesday 14 March 2006 14:46, Pete Chown wrote: Shannon -jj Behrens wrote: Arrows looks like a replacement for monads. Are you saying I should drop my use of the State monad? If so, why? I like the readability of the do syntax. Okay, now it's my turn to ask a question. :-) I've read about arrows, and while I think I see what they do, I'm not sure why they are seen as so special that they even get new syntax. This question of Shannon's is exactly the point I struggle with. I can see that the arrow operators might be useful with functions, but are they useful for other things too? Yes, http://www.haskell.org/arrows/biblio.html lists a number of papers describing non-trivial applications of Arrows, that is, Arrows other than (-). I found the exposition in http://www.haskell.org/yale/papers/oxford02/ to be quite readable. For example, as monads are one kind of arrow, I thought I would make some of the I/O functions into arrows and see what happened. The result was pretty much the same as using the monad, except slightly less convenient. You can write monadic code without ever using the syntax sugar, and get along. However, do-notation is convenient. OTOH, I am told that programming with Arrows is really quite inconvenient w/o the syntax sugar. Well, forgive me for my newbie-ness: o How important is it that I switch from using the State monad to using arrows? I can see no good reason to do it. o How important is it that I switch from using | or $ to using arrows? Not important. Arrows are just another way to structure a program. However, they have been designed for cases where a monad can /not/ be applied, such as e.g. self-optimizing parser combinators. (It seems that using arrows just to replace | or $ is like using a sledge hammer to drive a thumb tack.) Yes. o How much will this increase the conceptual complexity of my program--i.e. how much time am I going to have to spend explaining it in my article? A lot, so I'd say leave it alone. I would use either plain function application or --perhaps-- a state monad. o How much will this improve the readability or decrease the amount of code in my program? See above. I don't think you gain anything by using (). However, I still recommend using function application ($) instead of inverse application (|) because this closer to idiomatic Haskell. Besides, readability depends on how proficient the reader is. People who regularly program using Arrows may find it easy to read. I don't and have more difficulty understanding it than e.g. monadic code. Cheers, Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: request for code review
Shannon -jj Behrens wrote: o How important is it that I switch from using the State monad to using arrows? Your problem seems to be naturally soved by the State monad, therefore you should use that. o How important is it that I switch from using | or $ to using arrows? Unimportant. However, I'd recommend switching from application ($,|) to composition (.,) where possible. It's more functional and often easier to read. o How much will this increase the conceptual complexity of my program Not at all. You might define locally as f g = \x - g (f x) or just pretend that this definition is contained in Control.Arrow due to a historical accident, thereby completely ignoring the existence of other arrows. Udo. -- Wo die Macht geistlos ist, ist der Geist machtlos. (aus einem Gipfelbuch) signature.asc Description: Digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: request for code review
Ok, with all the various opinions, I think I'll: o Stick with the State monad. o Switch from | to $ and teach readers how to read it, Think of 'f $ g $ x' as 'f of g of x' or 'f(g(x))'. From that point of view, it may be helpful to read 'f $ g $ x' from right to left. Unless there are any objections, with that one change, I'll consider the coding done and move on to writing the article. Thanks so much for all of your various opinions and suggestions! I feel much more comfortable speaking from a position of authority knowing that all of you have reviewed my code! Best Regards, -jj On 3/15/06, Udo Stenzel [EMAIL PROTECTED] wrote: Shannon -jj Behrens wrote: o How important is it that I switch from using the State monad to using arrows? Your problem seems to be naturally soved by the State monad, therefore you should use that. o How important is it that I switch from using | or $ to using arrows? Unimportant. However, I'd recommend switching from application ($,|) to composition (.,) where possible. It's more functional and often easier to read. o How much will this increase the conceptual complexity of my program Not at all. You might define locally as f g = \x - g (f x) or just pretend that this definition is contained in Control.Arrow due to a historical accident, thereby completely ignoring the existence of other arrows. Udo. -- Wo die Macht geistlos ist, ist der Geist machtlos. (aus einem Gipfelbuch) -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.1 (GNU/Linux) iD8DBQFEF+f5c1ZCC9bsOpURAv2gAJwNirkt2yMFLlbTT9I2twUs3UcxdQCeKqx2 0FVTzx7VJEGtJexlGIJxero= =CPSW -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: request for code review
Shannon -jj Behrens wrote: I'm only using | as a replacement for $ because I find it more readable to read left to right than right to left. You can see this in two different ways, I think. Imagine the following: (+1) (*2) 3 This is not legal Haskell because it gets parsed as: ((+1) (*2)) 3 To avoid this problem, we can add our own brackets: (+1) ((*2) 3) Speaking loosely, $ is an alternative to the brackets, so we can also write: (+1) $ (*2) 3 We get the answer 7 whether we use brackets or $. If $ is going to be an alternative to brackets, we would be a bit surprised if the evaluation order changed. At the same time, it's true that if you think of this as a Unix pipe, the evaluation order is the wrong way round. We are evaluating right to left. Arrows are meant to be like Unix pipes. The whole idea is that you build up pipelines (and networks) of arrows. Usefully for you, functions are a kind of arrow, so you get the arrow operators automatically. As expected ((+1) (*2)) 3 gives 8 and not 7. Arrows looks like a replacement for monads. Are you saying I should drop my use of the State monad? If so, why? I like the readability of the do syntax. Okay, now it's my turn to ask a question. :-) I've read about arrows, and while I think I see what they do, I'm not sure why they are seen as so special that they even get new syntax. This question of Shannon's is exactly the point I struggle with. I can see that the arrow operators might be useful with functions, but are they useful for other things too? For example, as monads are one kind of arrow, I thought I would make some of the I/O functions into arrows and see what happened. The result was pretty much the same as using the monad, except slightly less convenient. I've been trying to use the arrow interface to HXT, but I don't see why it works better with arrows rather than functions. The arrows all do various transformations on the XML, but isn't that the idea of a function? Couldn't processTopDown, for example, be a function that maps an input XML tree to an output one, and takes a lambda expression which is to be applied to each node? Thanks, Pete ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: request for code review
On Tuesday 14 March 2006 14:46, Pete Chown wrote: Shannon -jj Behrens wrote: Arrows looks like a replacement for monads. Are you saying I should drop my use of the State monad? If so, why? I like the readability of the do syntax. Okay, now it's my turn to ask a question. :-) I've read about arrows, and while I think I see what they do, I'm not sure why they are seen as so special that they even get new syntax. This question of Shannon's is exactly the point I struggle with. I can see that the arrow operators might be useful with functions, but are they useful for other things too? Yes, http://www.haskell.org/arrows/biblio.html lists a number of papers describing non-trivial applications of Arrows, that is, Arrows other than (-). I found the exposition in http://www.haskell.org/yale/papers/oxford02/ to be quite readable. For example, as monads are one kind of arrow, I thought I would make some of the I/O functions into arrows and see what happened. The result was pretty much the same as using the monad, except slightly less convenient. You can write monadic code without ever using the syntax sugar, and get along. However, do-notation is convenient. OTOH, I am told that programming with Arrows is really quite inconvenient w/o the syntax sugar. Cheers, Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: request for code review
On 3/14/06, Benjamin Franksen [EMAIL PROTECTED] wrote: On Tuesday 14 March 2006 14:46, Pete Chown wrote: Shannon -jj Behrens wrote: Arrows looks like a replacement for monads. Are you saying I should drop my use of the State monad? If so, why? I like the readability of the do syntax. Okay, now it's my turn to ask a question. :-) I've read about arrows, and while I think I see what they do, I'm not sure why they are seen as so special that they even get new syntax. This question of Shannon's is exactly the point I struggle with. I can see that the arrow operators might be useful with functions, but are they useful for other things too? Yes, http://www.haskell.org/arrows/biblio.html lists a number of papers describing non-trivial applications of Arrows, that is, Arrows other than (-). I found the exposition in http://www.haskell.org/yale/papers/oxford02/ to be quite readable. For example, as monads are one kind of arrow, I thought I would make some of the I/O functions into arrows and see what happened. The result was pretty much the same as using the monad, except slightly less convenient. You can write monadic code without ever using the syntax sugar, and get along. However, do-notation is convenient. OTOH, I am told that programming with Arrows is really quite inconvenient w/o the syntax sugar. Well, forgive me for my newbie-ness: o How important is it that I switch from using the State monad to using arrows? o How important is it that I switch from using | or $ to using arrows? (It seems that using arrows just to replace | or $ is like using a sledge hammer to drive a thumb tack.) o How much will this increase the conceptual complexity of my program--i.e. how much time am I going to have to spend explaining it in my article? o How much will this improve the readability or decrease the amount of code in my program? Thanks! -jj ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: request for code review
Shannon -jj Behrens [EMAIL PROTECTED] writes: o How important is it that I switch from using the State monad to using arrows? Not at all. o How important is it that I switch from using | or $ to using arrows? Not at all. (It seems that using arrows just to replace | or $ is like using a sledge hammer to drive a thumb tack.) Exactly so. o How much will this increase the conceptual complexity of my program--i.e. how much time am I going to have to spend explaining it in my article? By a significant amount. Some might even argue that using monads is overkill... (Although in this case monads may indeed be justified.) o How much will this improve the readability or decrease the amount of code in my program? Not at all. Thanks! Not at all! :-) Regards, Malcolm ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: request for code review
Hi, I disagree with most people on this, since I am in general principle opposed to monads on the grounds that I don't understand them :) o How important is it that I switch from using the State monad to using arrows? I don't understand either monads or arrows o How important is it that I switch from using | or $ to using arrows? | is pure functional programming. $ is pure functional programming. It just so happens that the idiom you are more used to | is not the one most functional programmers are used to. If a solution can be done in a purely functional way, do so. If a function requires monads, use them. If you just want to do entirely functional things in a monadic way then use Java :) Disclaimer: I hate monads, everyone will disagree with me. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: request for code review
On 3/12/06, Einar Karttunen ekarttun@cs.helsinki.fi wrote: On 12.03 01:47, Shannon -jj Behrens wrote: monad. Perhaps controversially, I've continued to use | in a bunch of places that the monad didn't get rid of because I think it's more readable, but I'm still open for argument on this topic. Using the What about using () from Control.Arrow? -- For convenience: currTokType :: ParseContext - TokenType currTokType ctx = ctx | currTok | tokenType currTokType = currTok tokenType currTokValue :: ParseContext - String currTokValue ctx = ctx | currTok | tokenValue currTokValue = currTok tokenValue -- Create the final output string given a ParseContext. consolidateOutput :: ParseContext - String consolidateOutput ctx = ctx | output | reverse | concat consolidateOutput = output reverse concat and so on. I'm sorry, I looked at Arrow.hs, and I just don't understand. The State monad is working just fine. I'm only using | as a replacement for $ because I find it more readable to read left to right than right to left. Arrows looks like a replacement for monads. Are you saying I should drop my use of the State monad? If so, why? I like the readability of the do syntax. Are you saying that can be used as a reversed version of $? Thanks for your patiences with my ignorance ;) Thanks, -jj ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: request for code review
On 3/12/06, Lennart Augustsson [EMAIL PROTECTED] wrote:
Shannon -jj Behrens wrote:
lexString ('*':cs) = (classifyString *, cs)
lexString (c:cs) = (classifyString [c], cs)
The first line isn't needed, it does the same as the second line.
Good eye! You are correct.
Thanks,
-jj
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Re: [Haskell-cafe] Re: request for code review
On Mon, Mar 13, 2006 at 06:48:51PM -0800, Shannon -jj Behrens wrote: consolidateOutput = output reverse concat and so on. Are you saying that can be used as a reversed version of $? For the (-) instance of Arrow, () is simply reversed function composition, () = flip (.). Using Arrows for such a simple thing as function composition can be quite confusing, especially for beginners. Best regards Tomasz -- I am searching for programmers who are good at least in (Haskell || ML) (Linux || FreeBSD || math) for work in Warsaw, Poland ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: request for code review
Hi,
Thanks to everyone who reviewed my code and submitted comments the
first time! I've updated the code and transitioned to using the State
monad. Perhaps controversially, I've continued to use | in a bunch
of places that the monad didn't get rid of because I think it's more
readable, but I'm still open for argument on this topic. Using the
monad didn't make the code any shorter, but it kind of felt better,
once I figured out how to use it. Figuring out how to use execState
to get into and out of monad-ity was the hardest part, because it's
mentioned in so few of the examples. I think it's fair to say, of
course, that using a monad has increased the complexity, but I can
still read what I wrote. I've posted my code below for additional
comments.
Thanks again!
-jj
{- Translate C type declarations into English.
This exercise was taken from Expert C Programming: Deep C Secrets, p. 84.
Example: echo -n int *p; | runhugs cdecl.hs
Name: Shannon -jj Behrens [EMAIL PROTECTED]
Date: Fri Feb 17 00:03:38 PST 2006
-}
import Char (isSpace, isAlphaNum, isDigit)
import Control.Monad.State
-- | is like a UNIX pipe.
infixl 9 |
x | f = f x
data TokenType = Identifier | Qualifier | Type | Symbol Char
deriving (Show, Eq)
data Token = Token {
tokenType :: TokenType,
tokenValue :: String
} deriving Show
data ParseContext = ParseContext {
input :: String,-- The input that has not been parsed yet.
output :: [String], -- A list of strings in the reverse order of that which
-- they should be printed (e.g. [ a dog., I have]).
currTok :: Token, -- The current token, if defined.
stack :: [Token]-- A stack of tokens we haven't dealt with yet.
} deriving Show
-- For convenience:
currTokType :: ParseContext - TokenType
currTokType ctx = ctx | currTok | tokenType
currTokValue :: ParseContext - String
currTokValue ctx = ctx | currTok | tokenValue
-- Start a new State ParseContext given an input string.
createParseContext :: String - ParseContext
createParseContext input =
ParseContext {input = input, output = [], stack = []}
-- Create the final output string given a ParseContext.
consolidateOutput :: ParseContext - String
consolidateOutput ctx =
ctx | output | reverse | concat
-- Write to a ParseContext's output.
writeOutput :: String - State ParseContext ()
writeOutput s = modify (\ctx - ctx {output = s : output ctx})
-- Return the top token on the stack.
stackTop :: ParseContext - Token
stackTop ctx = ctx | stack | head
-- Pop the stack.
pop :: State ParseContext ()
pop = modify (\ctx - ctx {stack = ctx | stack | tail})
-- Write the value of the top of the stack and then pop it.
popAndWrite :: State ParseContext ()
popAndWrite = do
top - gets stackTop
writeOutput (tokenValue top)
pop
-- Classify a string into a Token.
classifyString :: String - Token
classifyString const = Token Qualifier read-only
classifyString * = Token (Symbol '*') pointer to
classifyString [c]
| not (isAlphaNum c) = Token (Symbol c) [c]
classifyString s= Token tokType s
where
tokType = case s of
volatile - Qualifier
x | x `elem` [void, char, signed, unsigned, short,
int, long, float, double, struct,
union, enum] - Type
x - Identifier
-- Read the next token into currTok.
getToken :: State ParseContext ()
getToken = modify getToken'
where
getToken' ctx@(ParseContext {input = s}) =
ctx {currTok = token, input = theRest}
where
(token, theRest) = s | lstrip | lexString
lstrip s = dropWhile isSpace s
-- Read a token. Return it and the left-over portion of the string.
lexString :: String - (Token, String)
lexString s@(c:cs) | isAlphaNum c = (token, theRest)
where
(tokString, theRest) = span isAlphaNum s
token = classifyString tokString
lexString ('*':cs) = (classifyString *, cs)
lexString (c:cs) = (classifyString [c], cs)
-- Put tokens on the stack until we reach the first identifier.
readToFirstIdentifier :: State ParseContext ()
readToFirstIdentifier = do
getToken
pushUntilIdentifier
afterIdentifier - get
let s = identifier ++ is
identifier = currTokValue afterIdentifier in
put (afterIdentifier {output = [s]})
getToken
-- Keep pushing tokens until we hit an identifier.
pushUntilIdentifier :: State ParseContext ()
pushUntilIdentifier = do
ctx - get
if currTokType ctx == Identifier
then return () -- Leave things as they are.
else do
put (ctx {stack = (currTok ctx) : (stack ctx)})
getToken
pushUntilIdentifier
return ()
-- Deal with arrays.
dealWithArrays :: State ParseContext ()
dealWithArrays = do
ctx - get
case currTokType ctx of
Symbol '[' - do
writeOutput array
getToken
writeIfNumber
getToken
writeOutput of
dealWithArrays
_ - return () -- Recurse until we get past the ['s.
where
writeIfNumber
[Haskell-cafe] Re: request for code review
On 12.03 01:47, Shannon -jj Behrens wrote: monad. Perhaps controversially, I've continued to use | in a bunch of places that the monad didn't get rid of because I think it's more readable, but I'm still open for argument on this topic. Using the What about using () from Control.Arrow? -- For convenience: currTokType :: ParseContext - TokenType currTokType ctx = ctx | currTok | tokenType currTokType = currTok tokenType currTokValue :: ParseContext - String currTokValue ctx = ctx | currTok | tokenValue currTokValue = currTok tokenValue -- Create the final output string given a ParseContext. consolidateOutput :: ParseContext - String consolidateOutput ctx = ctx | output | reverse | concat consolidateOutput = output reverse concat and so on. - Einar Karttunen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: request for code review
Shannon -jj Behrens wrote:
lexString ('*':cs) = (classifyString *, cs)
lexString (c:cs) = (classifyString [c], cs)
The first line isn't needed, it does the same as the second line.
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