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Today's Topics:
1. question on evaluation (Fabien R)
2. Re: question on evaluation (Theodore Lief Gannon)
----------------------------------------------------------------------
Message: 1
Date: Sat, 9 Jan 2016 16:21:46 +0100
From: Fabien R <[email protected]>
To: Haskell Beginners <[email protected]>
Subject: [Haskell-beginners] question on evaluation
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8; format=flowed
Hello,
I want to define a function blowup that takes "bang" as input and
returns "baannngggg".
I come up with these functions;
myReverse :: [a] -> [a]
myReverse [] = []
myReverse (x:xs) = myReverse xs ++ [x]
buildLenList :: String -> [Int]
buildLenList "" = []
buildLenList (_:xs) = [1 + length xs ] ++ buildLenList xs
myRepeat :: Char -> Int -> String
myRepeat x 0 = []
myRepeat x n = [x] ++ myRepeat x (n - 1)
blowup :: String -> String
blowup [] = []
blowup (x:xs) = myRepeat x (head ( (buildLenList (x:xs)))) ++ blowup xs
With this code, blowup "bang" returns "bbbbaaanng".
So I thought to insert myReverse between head and buildLenList but in
that case, the result is only "bang".
It seems that the evaluation of buildLenList is not working as I
thought. I tried to debug that using ghci debugger but failed (still
learning).
Can someone explain how the evaluation is done here ?
--
Fabien
------------------------------
Message: 2
Date: Sat, 9 Jan 2016 15:54:41 -0800
From: Theodore Lief Gannon <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] question on evaluation
Message-ID:
<CAJoPsuA_T7Je78YO-XScMFqrtWoT+EB84M8==Q_fb=r84ua...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Here's why you're getting the longer repetition first:
buildLenList "bang"
= buildLenList (_:"ang")
= [1 + length "ang"] ++ buildLenList "ang"
= [4] ++ buildLenList "ang"
(...) = [4, 3, 2, 1]
And the reason calling myReverse on that doesn't give you [1, 2, 3, 4] is
that you're reversing a different list every time:
blowup "bang"
= blowup ('b':"ang")
= myRepeat 'b' (head (myReverse (buildLenList ('b':"ang"))) ++ blowup "ang"
= myRepeat 'b' (head (myReverse [4, 3, 2, 1]) ++ blowup "ang"
= myRepeat 'b' (head [1, 2, 3, 4] ++ blowup "ang"
= myRepeat 'b' 1 ++ blowup "ang"
= "b" ++ blowup "ang"
= "b" ++ blowup ('a':"ng")
= "b" ++ myRepeat 'a' (head (myReverse (buildLenList ('a':"ng"))) ++ blowup
"ng"
= "b" ++ myRepeat 'a' (head (myReverse [3, 2, 1]) ++ blowup "ng"
= "b" ++ myRepeat 'a' (head [1, 2, 3] ++ blowup "ng"
(...)
Since you only asked for help with how it evaluates, I'll withhold further
spoilers. :) However, I strongly recommend getting more comfortable using
the (:) operator for construction instead of just pattern matching; in a
couple of your functions it would be a better tool than (++). Also,
"at-patterns" are great:
blowup l@(x:xs) = myRepeat x (head (buildLenList l)) ++ blowup xs
On Sat, Jan 9, 2016 at 7:21 AM, Fabien R <[email protected]> wrote:
> Hello,
>
> I want to define a function blowup that takes "bang" as input and returns
> "baannngggg".
> I come up with these functions;
>
> myReverse :: [a] -> [a]
> myReverse [] = []
> myReverse (x:xs) = myReverse xs ++ [x]
>
> buildLenList :: String -> [Int]
> buildLenList "" = []
> buildLenList (_:xs) = [1 + length xs ] ++ buildLenList xs
>
> myRepeat :: Char -> Int -> String
> myRepeat x 0 = []
> myRepeat x n = [x] ++ myRepeat x (n - 1)
>
> blowup :: String -> String
> blowup [] = []
> blowup (x:xs) = myRepeat x (head ( (buildLenList (x:xs)))) ++ blowup xs
>
> With this code, blowup "bang" returns "bbbbaaanng".
>
> So I thought to insert myReverse between head and buildLenList but in that
> case, the result is only "bang".
>
> It seems that the evaluation of buildLenList is not working as I thought.
> I tried to debug that using ghci debugger but failed (still learning).
> Can someone explain how the evaluation is done here ?
>
> --
> Fabien
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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