PS Stroustrup's comments about vectors are at best half right; push_back may extend the vector's length correctly, but operator[] on a vector certainly does not do bounds checking.

Sure it does, depending on how you configured the STL library. But this is off topic :)

Jonathan Cast wrote:

reverseDouble = unlines . intro . map show . reverse . map (read :: String -> Double) . takeWhile (/= "end") . words where intro l = ("read " ++ show (length l) ++ " elements") : "elements in reversed order" : l

This can be simplified to ... . map show . reverse . map read . ... = { map f . reverse = reverse . map f } ... . reverse . map show . map read . ... = { map f . map g = map (f . g) } ... . reverse . map (show . read) . ... = { show . read = id } ... . reverse . map id . ... = { map id = id } ... . reverse . ... In other words, reverseDouble = unlines. intro . reverse . takeWhile (/= "end") . words where intro xs = ("read " ++ show (length xs) ++ " elements") : "elements in reversed order" : xs And the doubles disappeared completely. :) Regards, H. Apfelmus

Eugene Kirpichov wrote:

Well, your program is not equivalent to the C++ version, since it doesn't bail on incorrect input.

Oops. That's because my assertion show . read = id is wrong. We only have read . show = id show . read <= id (in the "less defined than" sense) Regards, H. Apfelmus

On 17 jan 2009, at 22:22, Derek Elkins wrote:

On Thu, 2009-01-15 at 13:40 +0100, Apfelmus, Heinrich wrote:

...

Eugene Kirpichov wrote:

...

Well, your program is not equivalent to the C++ version, since it doesn't bail on incorrect input.

Oops. That's because my assertion

show . read = id

is wrong. We only have

read . show = id show . read <= id (in the "less defined than" sense)

No, you only have read . show = id which often doesn't hold in practice. show . read

You do not even have that; the read may remove surplus parentheses which will not be reinserted by the show. Doaitse

Assuming the first identity holds, you do of course have show . read . show = show and this probably holds even in most cases where read . show = id does not hold.

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Hello Eugene, Thursday, January 15, 2009, 3:27:59 PM, you wrote: but at least "length.map show" is eq to length :)

Well, your program is not equivalent to the C++ version, since it doesn't bail on incorrect input.

2009/1/15 Apfelmus, Heinrich :

...

Jonathan Cast wrote:

...

reverseDouble = unlines . intro . map show . reverse . map (read :: String -> Double) . takeWhile (/= "end") . words where intro l = ("read " ++ show (length l) ++ " elements") : "elements in reversed order" : l

This can be simplified to

... . map show . reverse . map read . ...

= { map f . reverse = reverse . map f }

... . reverse . map show . map read . ...

= { map f . map g = map (f . g) }

... . reverse . map (show . read) . ...

= { show . read = id }

... . reverse . map id . ...

= { map id = id }

... . reverse . ...

In other words,

reverseDouble = unlines. intro . reverse . takeWhile (/= "end") . words where intro xs = ("read " ++ show (length xs) ++ " elements") : "elements in reversed order" : xs

And the doubles disappeared completely. :)

Regards, H. Apfelmus

_______________________________________________ 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

-- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com