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" 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)
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)
On Fri, Dec 9, 2011 at 05:16, Yves Parès 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)
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 > On Fri, Dec 9, 2011 at 04:16, Yves Parès 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 04:16, Yves Parès 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)
Why do you people hate 'if' statements? 2011/12/9 Brandon Allbery > On Thu, Dec 8, 2011 at 15:52, Tom Murphy wrote: > >> On Wed, Dec 7, 2011 at 11:46 PM, Brandon Allbery wrote: >> >>> > 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 Thu, Dec 8, 2011 at 15:52, Tom Murphy wrote: > On Wed, Dec 7, 2011 at 11:46 PM, Brandon Allbery wrote: > >> > 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
Re: [Haskell-cafe] Why doesn't this work? (palindrome :: IO)
On Wed, Dec 7, 2011 at 11:46 PM, Brandon Allbery wrote: > On Wed, Dec 7, 2011 at 23:24, Alexej Segeda > 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)
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 23:24, Alexej Segeda 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" (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
[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
