<div dir="ltr">Think about what the possible values of "xs" might be, and trace through the next call to "plusplus xs ys".<div><br></div><div>It would help if I didn't name the variables stupidly... here is a slightly better version:</div><div><br></div><div>plusplus [] ys = ys</div><div>plusplus (x:xs) ys = x : plusplus xs ys</div><div><br></div><div>If it helps, try tracing through the steps required to evaluate "plusplus [1, 2] [3, 4]" manually (using a pen and paper, not on the computer)</div></div><div class="gmail_extra"><br><div class="gmail_quote">2015-05-13 15:55 GMT+09:00 Roelof Wobben <span dir="ltr"><<a href="mailto:r.wobben@home.nl" target="_blank">r.wobben@home.nl</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
<div>Thanks, <br>
<br>
So the answer is x and the rest of xs and ys. <br>
How do x then get added to ys. <br>
<br>
Roelof<br>
<br>
Daniel P. Wright schreef op 13-5-2015 om 8:52:<br>
</div><div><div class="h5">
<blockquote type="cite">
<div dir="ltr">Ah, that's just a small syntactic issue -- in
Haskell, operators are infix (go between the arguments) by
default, but named functions are not. So you would have to
write it:
<div><br>
</div>
<div>plusplus [] xs = xs</div>
<div>plusplus (x:xs) ys = x : plusplus xs ys</div>
<div><br>
</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">2015-05-13 15:39 GMT+09:00 Roelof
Wobben <span dir="ltr"><<a href="mailto:r.wobben@home.nl" target="_blank">r.wobben@home.nl</a>></span>:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
<div>Thanks, <br>
<br>
If I re-implement it like this : <br>
<span> <br>
plusplus :: [a] -> [a] -> [a]<br>
plusplus [] (xs) = xs <br>
</span> plusplus (x:xs) yx = x : xs plusplus yx<br>
<br>
main = print $ ["a","b"] plusplus ["c","d"]<br>
<br>
<br>
I see this error appear : <br>
<br>
<div>src/Main.hs@3:26-3:40 </div>
<div><span>Couldn't match expected type ‘([a0] ->
[a0] -> [a0]) -> [a] -> [a]’ with actual
type </span>
<div style="font-size:14px">[<span>a</span>]</div>
<span> Relevant bindings include yx :: [a] (bound at
/home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:3:17)
xs :: [a] (bound at
/home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:3:13)
x :: a (bound at
/home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:3:11)
plusplus :: [a] -> [a] -> [a] (bound at
/home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:2:1)
The
function </span>
<div style="font-size:14px"><span>xs</span></div>
<span> is applied to two arguments, but its type </span>
<div style="font-size:14px">[<span>a</span>]</div>
<span> has none</span><span title="Click to show/hide
extra information"> …<br>
<br>
So for me not a aha moment. I was hoping I would
get it <br>
<br>
Roelof</span><br>
</div>
<br>
Daniel P. Wright schreef op 13-5-2015 om 8:27:<br>
</div>
<div>
<div>
<blockquote type="cite">
<div dir="ltr">Hi Roelof,
<div><br>
</div>
<div>If you don't consider it cheating (and I
suggest you shouldn't, having had a stab at the
answers), you will find great enlightenment
looking at how these functions are *actually*
implemented in the wild. Did you know that
there is a "source" link for each function on
Hackage? By clicking on that, for your first
example, you can compare the actual
implementation of (++) with your "plusplus"
function:</div>
<div><br>
</div>
<div><a href="http://hackage.haskell.org/package/base-4.8.0.0/docs/src/GHC-Base.html#%2B%2B" target="_blank">http://hackage.haskell.org/package/base-4.8.0.0/docs/src/GHC-Base.html#%2B%2B</a><br>
</div>
<div><br>
</div>
<div>(By the way, it's that function in particular
that gave me one of my first "Aha!" moments in
Haskell... it's quite beautiful in its
simplicity).</div>
<div><br>
</div>
<div>One thing with viewing the source on Hackage
is that sometimes it can be a little more
confusing than it needs to be for the sake of
efficiency. A really good source for good,
readable examples of Prelude functions in
Haskell is the Haskell Report:</div>
<div><br>
</div>
<div><a href="https://www.haskell.org/onlinereport/standard-prelude.html" target="_blank">https://www.haskell.org/onlinereport/standard-prelude.html</a></div>
<div><br>
</div>
<div>(In this case, though, the implementation is
the same).</div>
<div><br>
</div>
<div>Having a shot at defining these library
functions yourself, as you have done, and then
comparing your version with the "official"
version in the prelude is a great way to learn
good style!</div>
<div><br>
</div>
<div>-Dani. </div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">2015-05-13 15:10
GMT+09:00 Roelof Wobben <span dir="ltr"><<a href="mailto:r.wobben@home.nl" target="_blank">r.wobben@home.nl</a>></span>:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hello,<br>
<br>
For practising pattern matching and recursion
I did recreate some commands of Data,list.<br>
<br>
My re-implementation of ++ :<br>
<br>
plusplus :: [a] -> [a] -> [a]<br>
plusplus [] [] = [] ;<br>
plusplus [] (xs) = xs<br>
plusplus (xs) [] = xs<br>
plusplus (xs) yx = plusplus' (reverse xs) yx<br>
<br>
plusplus' :: [a] -> [a] -> [a]<br>
plusplus' [] (xs) = xs<br>
plusplus' (x:xs) yx = plusplus' xs (x:yx)<br>
<br>
main = print $ plusplus ["a","b"] ["c","d"]<br>
<br>
my re-implementation of init :<br>
<br>
import Data.Maybe<br>
<br>
-- | The main entry point.<br>
init' :: [a] -> Maybe [a]<br>
init' [] = Nothing<br>
init' [x] = Just []<br>
init' (x:xs) = Just (x:fromMaybe xs (init'
xs))<br>
<br>
main = print . init' $ [1,3]<br>
<br>
<br>
my re-implementation of last :<br>
<br>
-- | The main entry point.<br>
last' :: [a] -> Maybe a<br>
last' [] = Nothing<br>
last' [x] = Just x<br>
last' (_:xs) = last' xs<br>
<br>
main = print . last' $ []<br>
<br>
Now I wonder if these solutions are the
haskell way ? if not so, how can I improve
them ,<br>
<br>
Roelof<br>
<br>
<br>
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