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<div class="moz-cite-prefix">Hello, <br>
<br>
I now also see that error.<br>
<br>
What I try to do is : <br>
<br>
pp' [] xs = xs when the first list is empty then the output is
the second list<br>
pp' [] [] = [] when both list are empty then the output will be a
empty array. <br>
<br>
I flipped both rules and now the error is gone. <br>
Now I hope to find out how I can take care that the right number
is added to the second list <br>
<br>
Roelof<br>
<br>
Jonathan Skårstedt schreef op 12-5-2015 om 20:58:<br>
</div>
<blockquote
cite="mid:CAD6iYwYKfEKx8nNRwco6KinkOK_3wgG6Qy2i073pLO=_Nqm5gw@mail.gmail.com"
type="cite">
<div dir="ltr">
<div>Hello!</div>
<div><br>
</div>
<div>I will call plusplus' by pp' in the text below<br>
</div>
<div><br>
</div>
<div>1. Your pp' gives a warning, namely that the patterns </div>
<div>pp' [] xs = xs</div>
<div>pp' [] [] = []</div>
<div>are overlapping. This is because of pattern matching
working top-down ie if I give the input <b>pp' [] [] </b>it
will match both your function entry points at <b>pp' [] xs</b>
and <b>pp' [] []</b>. We don't really need <b>pp' [] []</b>
as <b>pp' [] ys = ys </b>will give the correct answer even
if ys is an empty list.<br>
</div>
<div><br>
</div>
<div>2. In your last function body you have this expression </div>
<div>pp' (x:xs) ys = pp' xs (x:ys) </div>
<div><br>
</div>
<div>this will not do what you expect, consider the flow below.</div>
<div><br>
</div>
<div>I will call pp' on two strings "ABC" and "DEF"</div>
<div><br>
</div>
<div>Lets extend this function</div>
<div>pp' (A:BC) DEF = pp' BC (A:DEF)</div>
<div>pp' (B:C) ADEF = pp' C (B:ADEF)</div>
<div>pp' C:[] BADEF = pp' [] (C:BADEF)</div>
<div>pp' [] CBADEF = CBADEF -- <- base case</div>
<div><br>
</div>
<div>As you see, it will pick the first of xs to put first on ys
but is this what we want? For one recursion, yes but after
that it gets a bit weird. The second recursion will flip the
order of the resulting list by adding the first object at the
start first, instead of last.</div>
<div><br>
</div>
<div>We actually don't want our construction (the colon :) to
work from inside the recursive call (pp' xs (x:ys)), we want
to move it outside, like this, and just like we did in init:
pp' (x:xs) ys = <b>x: </b>pp' xs ys</div>
<div><br>
</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">2015-05-12 20:33 GMT+02:00 Roelof
Wobben <span dir="ltr"><<a moz-do-not-send="true"
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>pfff, I got grazy here <br>
<br>
On this function it works fine : <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>
and on this one it does not work : <br>
<span class=""> <br>
plusplus' :: [a] -> [a] -> [a]<br>
plusplus' [] (xs) = xs <br>
plusplus' [] [] = [] <br>
plusplus' (xs) [] = xs<br>
</span> plusplus' (x:xs) yx = plusplus' xs (x:yx)<br>
<br>
<br>
main = print . plusplus' $ [1,2] [3, 4]<br>
<br>
Roelof<br>
<br>
<br>
<br>
<br>
Alex Hammel schreef op 12-5-2015 om 20:19:<br>
</div>
<div>
<div class="h5">
<blockquote type="cite">
<div dir="ltr">
<div>The relevant part of that error message is
this:<br>
<br>
<span>> The function ‘[1, 2]’ is applied to
one argument, but its type </span>
<div style="font-size:14px">> [<span>t0</span>]<br>
</div>
<div style="font-size:14px">> has none<br>
</div>
<br>
</div>
Which means that you're trying to call a list
literal as though it were a function (with one
argument). Which probably means that you've got a
($) out of place.<br>
<div>
<div>
<div><br>
</div>
</div>
</div>
</div>
<br>
<div class="gmail_quote">On Tue, 12 May 2015 at
11:16 Roelof Wobben <<a moz-do-not-send="true"
href="mailto:r.wobben@home.nl" target="_blank">r.wobben@home.nl</a>>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0
0 .8ex;border-left:1px #ccc
solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
<div>Thanks, <br>
<br>
Now I see the last error on the main line :
<br>
<br>
<div>src/Main.hs@7:28-7:39 </div>
<div><span>Couldn't match expected type
‘[t1] -> [a0]’ with actual type </span>
<div style="font-size:14px">[<span>t0</span>]</div>
<span> The function ‘[1, 2]’ is applied to
one argument, but its type </span>
<div style="font-size:14px">[<span>t0</span>]</div>
<span> has none</span><span title="Click
to show/hide extra information"> …</span><span
style="display:inline"> In the second
argument of ‘($)’, namely ‘[1, 2] [3,
4]’ In the expression: print . plusplus'
$ [1, 2] [3, 4]<br>
<br>
Roelof<br>
<br>
</span></div>
<br>
<br>
<br>
Alex Hammel schreef op 12-5-2015 om 20:11:<br>
</div>
</div>
<div bgcolor="#FFFFFF" text="#000000">
<blockquote type="cite">
<div dir="ltr">
<div>You're missing the parens around
(x:yx) in the last clause.<br>
</div>
</div>
<br>
<div class="gmail_quote">On Tue, 12 May 2015
at 11:02 Roelof Wobben <<a
moz-do-not-send="true"
href="mailto:r.wobben@home.nl"
target="_blank">r.wobben@home.nl</a>>
wrote:<br>
<blockquote class="gmail_quote"
style="margin:0 0 0 .8ex;border-left:1px
#ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
Hello, <br>
<br>
I try to re implement ++ <br>
<br>
So far I have this ; <br>
<br>
plusplus' :: [a] -> [a] -> [a]<br>
plusplus' [] (xs) = xs <br>
plusplus' [] [] = [] <br>
plusplus' (xs) [] = xs<br>
plusplus' (x:xs) yx = plusplus' xs
x:yx<br>
<br>
main = print . plusplus' $ [1,2] [3,4]<br>
<br>
<br>
But I still get this error message : <br>
<br>
<div>src/Main.hs@5:23-5:37 </div>
<div><span>Couldn't match expected
type </span>
<div style="font-size:14px"><span>a</span></div>
<span> with actual type </span>
<div style="font-size:14px">[<span>a</span>]</div>
<span> </span>
<div style="font-size:14px"><span>a</span></div>
<span> is a rigid type variable
bound by the type signature for
plusplus' :: [a] -> [a] ->
[a] at
/home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:1:14
Relevant
bindings include yx :: [a] (bound
at
/home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:5:18)
xs :: [a] (bound at
/home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:5:14)
x :: a (bound at
/home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:5:12)
plusplus' :: [a] -> [a] ->
[a] (bound at
/home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:2:1)</span><span
title="Click to show/hide extra
information"> …<br>
<br>
<br>
</span></div>
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-- <br>
<div class="gmail_signature">Jonathan Skårstedt<br>
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