Re: [Haskell-cafe] Why doesn't this work? (palindrome :: IO)
Brandon Allbery wrote: case () of () | s == reverse s - putStrLn palindrome _ - putStrLn nope Tom Murphy wrote: This is kind of a hack of case, though. I think what the OP was looking for is isPalindrome word | (word == reverse word) = putStrLn (word ++ is a palindrome) | otherwise = putStrLn (word ++ is not a palindrome) Erm? It's as much of a hack of case as yours is, since the above is actually using case. I agree with Tom here. While it's true that the compiler internally desugars to case, that low-level compiler transformation doesn't have much to do with the best way to write clear code. I find that case often creates code that is more confusing and bug-prone. Except when what I really want to express is pattern matching, *and* there is some specific reason here why I don't want to use a named function in a let or where binding. Altogether, it doesn't come up very often for me. And even for styles that use case more than I do, certainly there is room to call the use of the case () idiom a hack. (Even though I'll admit that I do use it sometimes.) Regards, Yitz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why doesn't this work? (palindrome :: IO)
To clarify, by hack I meant that it seemed like a workaround specifically to keep case in the OP's code, when it seemed like they were looking for the functionality of guards. amindfv / Tom On Dec 11, 2011 1:39 PM, Yitzchak Gale g...@sefer.org wrote: Brandon Allbery wrote: case () of () | s == reverse s - putStrLn palindrome _ - putStrLn nope Tom Murphy wrote: This is kind of a hack of case, though. I think what the OP was looking for is isPalindrome word | (word == reverse word) = putStrLn (word ++ is a palindrome) | otherwise = putStrLn (word ++ is not a palindrome) Erm? It's as much of a hack of case as yours is, since the above is actually using case. I agree with Tom here. While it's true that the compiler internally desugars to case, that low-level compiler transformation doesn't have much to do with the best way to write clear code. I find that case often creates code that is more confusing and bug-prone. Except when what I really want to express is pattern matching, *and* there is some specific reason here why I don't want to use a named function in a let or where binding. Altogether, it doesn't come up very often for me. And even for styles that use case more than I do, certainly there is room to call the use of the case () idiom a hack. (Even though I'll admit that I do use it sometimes.) Regards, Yitz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why doesn't this work? (palindrome :: IO)
Why do you people hate 'if' statements? 2011/12/9 Brandon Allbery allber...@gmail.com On Thu, Dec 8, 2011 at 15:52, Tom Murphy amin...@gmail.com wrote: On Wed, Dec 7, 2011 at 11:46 PM, Brandon Allbery allber...@gmail.comwrote: case () of () | s == reverse s - putStrLn palindrome _ - putStrLn nope This is kind of a hack of case, though. I think what the OP was looking for is isPalindrome word | (word == reverse word) = putStrLn (word ++ is a palindrome) | otherwise = putStrLn (word ++ is not a palindrome) Erm? It's as much of a hack of case as yours is, since the above is actually using case. -- brandon s allbery allber...@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms ___ 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] Why doesn't this work? (palindrome :: IO)
On Fri, Dec 9, 2011 at 04:16, Yves Parès limestr...@gmail.com wrote: Why do you people hate 'if' statements? It's more that the language spec does; if statements, along with a number of other things, desugar to case which is the fundamental conditional construct. (And more personally, I find the indentation behavior annoying, in that I need to indent then and else more inside something that uses layout; plus the lack of an else if that is aware of being part of a compound means that it ladders even more, so I end up switching to case just to keep from ending up on column 200 or something.) -- brandon s allbery allber...@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why doesn't this work? (palindrome :: IO)
I agree with all that, but in *this *special case, I think that case something of True - False - is less nice and obvious than if something then else 2011/12/9 Brandon Allbery allber...@gmail.com On Fri, Dec 9, 2011 at 04:16, Yves Parès limestr...@gmail.com wrote: Why do you people hate 'if' statements? It's more that the language spec does; if statements, along with a number of other things, desugar to case which is the fundamental conditional construct. (And more personally, I find the indentation behavior annoying, in that I need to indent then and else more inside something that uses layout; plus the lack of an else if that is aware of being part of a compound means that it ladders even more, so I end up switching to case just to keep from ending up on column 200 or something.) -- brandon s allbery allber...@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why doesn't this work? (palindrome :: IO)
On Fri, Dec 9, 2011 at 05:16, Yves Parès limestr...@gmail.com wrote: I agree with all that, but in *this *special case, I think that I should also note that the OP mentioned using if, but was surprised/confused by the behavior of case, which is why that's what we've been focusing on. -- brandon s allbery allber...@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why doesn't this work? (palindrome :: IO)
Alexej The interesting thing is, that if I change the case ... of Alexej statement to an if ... then ... else statement, this magically Alexej starts to work. Since I no longer am enrolled (I have to take Alexej the course next year), I can't ask a teacher, but my curiosity Alexej still bugs me. Why doesn't this work? And why does it work with Alexej a if ... then ...else statement? maybe you mixed up 'if' and 'case' usages. In fact, 'if' can alway be translated to 'case' by matching on the boolean condition, like below : case (s == reverse s) of True - s is a palindrome False - s is not a palindrome -- Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why doesn't this work? (palindrome :: IO)
On Wed, Dec 7, 2011 at 11:46 PM, Brandon Allbery allber...@gmail.comwrote: On Wed, Dec 7, 2011 at 23:24, Alexej Segeda aloscha_den_st...@hotmail.com wrote: case s of (s == reverse s)- putStrLn (s ++ is a palindrome) otherwise - putStrLn (s ++ is not a palindrome) case does pattern matching, not Boolean expressions. (s == reverse s) is not a useful pattern, and in fact is probably a syntax error because ==is not a valid infix constructor. If you want to do Boolean comparisons in a case, you need to use something like case () of () | s == reverse s - putStrLn palindrome _ - putStrLn nope This is kind of a hack of case, though. I think what the OP was looking for is palindrome :: IO () palindrome = do putStrLn Type in a word s - getLine isPalindrome s isPalindrome word | (word == reverse word) = putStrLn (word ++ is a palindrome) | otherwise = putStrLn (word ++ is not a palindrome) amindfv / Tom ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why doesn't this work? (palindrome :: IO)
On Thu, Dec 8, 2011 at 15:52, Tom Murphy amin...@gmail.com wrote: On Wed, Dec 7, 2011 at 11:46 PM, Brandon Allbery allber...@gmail.comwrote: case () of () | s == reverse s - putStrLn palindrome _ - putStrLn nope This is kind of a hack of case, though. I think what the OP was looking for is isPalindrome word | (word == reverse word) = putStrLn (word ++ is a palindrome) | otherwise = putStrLn (word ++ is not a palindrome) Erm? It's as much of a hack of case as yours is, since the above is actually using case. -- brandon s allbery allber...@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Why doesn't this work? (palindrome :: IO)
Hi! A couple of months ago, I wrote an exam in an introductory Haskell course and failed, all because of an assignment that I was convinced would work, but for some reason, it didn't. The assignment was to write a function that would take a line, then determine whether it's a palindrome or not. My code follows: palindrome :: IO () palindrome = do putStr Type in a word s - getLine case s of (s == reverse s)- putStrLn (s ++ is a palindrome) otherwise - putStrLn (s ++ is not a palindrome) The interesting thing is, that if I change the case ... of statement to an if ... then ... else statement, this magically starts to work. Since I no longer am enrolled (I have to take the course next year), I can't ask a teacher, but my curiosity still bugs me. Why doesn't this work? And why does it work with a if ... then ...else statement? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why doesn't this work? (palindrome :: IO)
On Wed, Dec 7, 2011 at 23:24, Alexej Segeda aloscha_den_st...@hotmail.comwrote: case s of (s == reverse s)- putStrLn (s ++ is a palindrome) otherwise - putStrLn (s ++ is not a palindrome) case does pattern matching, not Boolean expressions. (s == reverse s) is not a useful pattern, and in fact is probably a syntax error because == is not a valid infix constructor. If you want to do Boolean comparisons in a case, you need to use something like case () of () | s == reverse s - putStrLn palindrome _ - putStrLn nope (otherwise isn't doing what you think there, either; it's exactly equivalent to the _ (unnamed placeholder) I used, since you aren't then using otherwise as the local binding (shadowing the Prelude one) that it is.) -- brandon s allbery allber...@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
