[Haskell-beginners] Beginners Digest, Vol 122, Issue 19
Andrey Klaus
deepmindster at gmail.com
Mon Aug 27 21:55:48 UTC 2018
Hello everybody,
I do not have many experience with haskell, but I'm programming for more
then 10 years
and I'm spending almost all my free time to learn FP during last 1.5 years.
I have read book being discussed (Graham Hutton) and I would like to say
just a couple words about it.
First of all you need remember that it is book for beginners, so, IMO
author would prefer
to use "possible easy to understand" way instead of "most type safe" or
"most canonical".
And this is good for beginners because otherwise details take too many time
with
unreasanable low results.
Hutton's style is very understandable. No even one needless detail over all
the book.
Code looks to me simple, concise and very clean. Despite it looks not safe
sometimes, it is always perfectly reasonable. I want to say I would be happy
if I were to write such code in production.
When one has his own experience in haskell, she or he will be able to
choose his own style.
But for beginners I would surely recommend to follow Hutton's style.
wbr,
Andrey
пн, 27 авг. 2018 г. в 15:41, <beginners-request at haskell.org>:
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> 1. Re: Hutton ex 7.7 and 7.8 (C Maeder)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 27 Aug 2018 10:02:48 +0200
> From: C Maeder <chr.maeder at web.de>
> To: fa-ml at ariis.it
> Cc: The Haskell-Beginners Mailing List - Discussion of primarily
> beginner-level topics related to Haskell <Beginners at haskell.org>
> Subject: Re: [Haskell-beginners] Hutton ex 7.7 and 7.8
> Message-ID: <7c43aeb1-80ea-5bc5-e683-002cfeeea796 at web.de>
> Content-Type: text/plain; charset=utf-8
>
> Hi, see my comments below
>
> Am 24.08.2018 um 11:45 schrieb trent shipley:
> >
> > I am mostly looking for style feedback, although if there are any
> > obvious logic errors, I'd be "happy" to learn about those too.
>
> [...] let me ignore the exercise text
>
> >
> > type Bit = Int
>
> A bit is not an Int! A user-defined type (or Bool) would be type safer
> (although arithmetic is missing).
>
> >
> > byte :: Int
> > byte = 8
> >
> > parityByte :: Int
> > parityByte = 9
> >
> > bin2int :: [Bit] -> Int
> > bin2int = foldr (\x y -> x + 2 * y) 0
>
> use here "bit2int x" if bits are no ints.
>
> >
> > -- Hutton, Graham. Programming in Haskell (Kindle Locations 2647-2649).
> > Cambridge University Press. Kindle Edition.
> >
> > int2bin :: Int -> [Bit]
> > int2bin 0 = []
> > int2bin n = n `mod` 2 : int2bin (n `div` 2)
>
> here you could use "even n" if bits are Bool.
>
> >
> > -- Hutton, Graham. Programming in Haskell (Kindle Locations 2654-2656).
> > Cambridge University Press. Kindle Edition.
> >
> > make8 :: [Bit] -> [Bit]
> > make8 bits = addParity (take byte (bits ++ repeat 0))
> >
> > -- Parity functions
> >
> > addParity :: [Bit] -> [Bit]
> > addParity xs = if even (sum xs)
> > then xs ++ [0]
> > else xs ++ [1]
>
> appending the parity (at the end of a list) is inefficient. The parity
> could be the first bit if it is not stored separately.
>
> >
> > checkParity :: [Bit] -> Bool
> > checkParity xs = (((even . sum) (take ((length xs) - 1) xs)) ==
> > ((even . last) xs)) ||
> > (((odd . sum) (take ((length xs) - 1) xs)) ==
> > ((odd . last) xs))
>
> here is too much duplicate code! You could also use the function "init",
> if the input list xs is (checked to be) non-empty.
>
> >
> > errorParity :: [Bit] -> ([Bit], Bool)
> > errorParity xs = if checkParity xs
> > then (xs, checkParity xs)
> > else error "Parity error"
>
> the result of this function is nonsense. The input (usually) does not
> need to be returned and the boolean result can only be true, since the
> other case fails with a runtime error.
>
> >
> > dropParity :: [Bit] -> [Bit]
> > dropParity xs = take ((length xs) - 1) xs
>
> this function could have been reused in checkParity.
>
> >
> > -- Hutton, Graham. Programming in Haskell (Kindle Locations 2662-2663).
> > Cambridge University Press. Kindle Edition.
> >
> > -- TRANSMISSION
> >
> > encode :: String -> [Bit]
> > encode = concat . map (make8 . int2bin . ord)
> >
> > -- Hutton, Graham. Programming in Haskell (Kindle Locations 2673-2675).
> > Cambridge University Press. Kindle Edition.
> >
> > chop8 :: [Bit] -> [[Bit]]
> > chop8 [] = []
> > chop8 bits = (dropParity . fst . errorParity) (take parityByte bits) :
> > chop8 (drop parityByte bits)
>
> use "splitAt" instead of take and drop. (It may not be very nice to fail
> with a runtime error by using errorParity.)
>
> >
> > -- Hutton, Graham. Programming in Haskell (Kindle Locations 2681-2683).
> > Cambridge University Press. Kindle Edition.
> >
> > decode :: [Bit] -> String
> > decode = map (chr . bin2int) . chop8
> >
> > -- Hutton, Graham. Programming in Haskell (Kindle Locations 2686-2688).
> > Cambridge University Press. Kindle Edition.
> >
> > -- channel :: [Bit] -> [Bit]
> > -- channel = id
> >
> > channel :: [Bit] -> [Bit]
> > channel = tail
>
> "tail" is partial! This should be documented (if this is ok). Did you
> try to transmit an empty string?
>
> Cheers Christian
>
> >
> > -- Hutton, Graham. Programming in Haskell (Kindle Locations 2696-2697).
> > Cambridge University Press. Kindle Edition.
> >
> > transmit :: String -> String
> > transmit = decode . channel . encode
> >
> > -- Hutton, Graham. Programming in Haskell (Kindle Locations 2694-2695).
> > Cambridge University Press. Kindle Edition.
> >
> >
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> > Beginners at haskell.org
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> >
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