<div dir="auto">[x] means a list containing the element x<div dir="auto"><br><div dir="auto">xs in general is only a variable named xs, and in particular could be a list; in this case, if you write</div><div dir="auto">first@(x:xs)</div><div dir="auto">you mean that first is a list, its "head" is x and xs is its "tail", i.e. all the elements of the list first following x.</div><div dir="auto"><br></div><div dir="auto">For example if the arguments of merge are [1,2,4] and [3,6], then</div><div dir="auto">first = [1,2,4]</div><div dir="auto">x=1</div><div dir="auto">xs = [2,4]</div><div dir="auto">second=[3,6]</div><div dir="auto">y=3</div><div dir="auto">ys=[6]</div><div dir="auto"><br></div><div dir="auto">Note that [1,2,4] could not be pattern-matchd with [x]. [6] could (because contains just one element)</div></div></div><br><div class="gmail_quote"><div dir="ltr">Il ven 18 mag 2018, 08:04 trent shipley <<a href="mailto:trent.shipley@gmail.com">trent.shipley@gmail.com</a>> ha scritto:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Thanks to all. I used Mukesh's suggestion. <div><br></div><div>I am still not clear on:</div><div><br></div><div>why <font face="monospace" size="2">[x] /= xs</font></div><div>why <font face="monospace" size="2">first == first@(x:xs)</font><font size="2">, especially weather the variable declarations are considered names for the same thing.</font></div></div><br><div class="gmail_quote"><div dir="ltr">On Thu, May 17, 2018 at 10:39 PM Hemanth Gunda <<a href="mailto:hemanth.420@gmail.com" target="_blank" rel="noreferrer">hemanth.420@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Hi Trent, <div><br></div><div>This works: </div></div><div dir="ltr"><div><br></div><div>
<div style="color:rgb(248,248,242);background-color:rgb(39,40,34);font-family:Consolas,"Courier New",monospace;font-weight:normal;font-size:14px;line-height:19px;white-space:pre-wrap"><div><span style="color:rgb(166,226,46)">merge</span><span style="color:rgb(249,38,114)">::</span><span style="color:rgb(248,248,242)"> </span><span style="color:rgb(102,217,239);font-style:italic">Ord</span><span style="color:rgb(248,248,242)"> </span><span style="color:rgb(248,248,242)">a</span><span style="color:rgb(248,248,242)"> </span><span style="color:rgb(249,38,114)">=></span><span style="color:rgb(248,248,242)"> [</span><span style="color:rgb(248,248,242)">a</span><span style="color:rgb(248,248,242)">] </span><span style="color:rgb(249,38,114)">-></span><span style="color:rgb(248,248,242)"> [</span><span style="color:rgb(248,248,242)">a</span><span style="color:rgb(248,248,242)">] </span><span style="color:rgb(249,38,114)">-></span><span style="color:rgb(248,248,242)"> [</span><span style="color:rgb(248,248,242)">a</span><span style="color:rgb(248,248,242)">]</span></div><div><span style="color:rgb(248,248,242)">merge </span><span style="color:rgb(174,129,255)">[]</span><span style="color:rgb(248,248,242)"> </span><span style="color:rgb(174,129,255)">[]</span><span style="color:rgb(248,248,242)"> </span><span style="color:rgb(249,38,114)">=</span><span style="color:rgb(248,248,242)"> </span><span style="color:rgb(174,129,255)">[]</span></div><div><span style="color:rgb(248,248,242)">merge x </span><span style="color:rgb(174,129,255)">[]</span><span style="color:rgb(248,248,242)"> </span><span style="color:rgb(249,38,114)">=</span><span style="color:rgb(248,248,242)"> x</span></div><div><span style="color:rgb(248,248,242)">merge </span><span style="color:rgb(174,129,255)">[]</span><span style="color:rgb(248,248,242)"> y </span><span style="color:rgb(249,38,114)">=</span><span style="color:rgb(248,248,242)"> y</span></div><div><span style="color:rgb(248,248,242)">merge first</span><span style="color:rgb(249,38,114)">@</span><span style="color:rgb(248,248,242)">(x</span><span style="color:rgb(249,38,114)">:</span><span style="color:rgb(248,248,242)">xs) second</span><span style="color:rgb(249,38,114)">@</span><span style="color:rgb(248,248,242)">(y</span><span style="color:rgb(249,38,114)">:</span><span style="color:rgb(248,248,242)">ys) </span></div><div><span style="color:rgb(248,248,242)"> </span><span style="color:rgb(249,38,114)">|</span><span style="color:rgb(248,248,242)"> x </span><span style="color:rgb(249,38,114)"><=</span><span style="color:rgb(248,248,242)"> y </span><span style="color:rgb(249,38,114)">=</span><span style="color:rgb(248,248,242)"> x </span><span style="color:rgb(249,38,114)">:</span><span style="color:rgb(248,248,242)"> merge xs second </span></div><div><span style="color:rgb(248,248,242)"> </span><span style="color:rgb(249,38,114)">|</span><span style="color:rgb(248,248,242)"> otherwise </span><span style="color:rgb(249,38,114)">=</span><span style="color:rgb(248,248,242)"> y </span><span style="color:rgb(249,38,114)">:</span><span style="color:rgb(248,248,242)"> merge first ys</span></div></div>
<br></div></div><div dir="ltr"><div>Difference in the lines</div></div><div dir="ltr"><div><br></div><div>
<div style="font-family:Consolas,"Courier New",monospace;font-size:14px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:pre-wrap;word-spacing:0px;text-decoration-style:initial;text-decoration-color:initial"><font color="#000000">merge x [] = x</font></div><div style="font-family:Consolas,"Courier New",monospace;font-size:14px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:pre-wrap;word-spacing:0px;text-decoration-style:initial;text-decoration-color:initial"><font color="#000000">merge [] y = y</font></div></div><div><br></div></div><div dir="ltr"><div>As the input is of type [a] where a belongs to typeclass Ord, you must pass x instead of [x]. </div><div><br></div><div>[x] would work if you tried merge [4] []. but will fail if you tried merge [4,5] []. because "4,5" isn't of type a. </div><div><br></div><div>Regards, Hemanth</div><div><br></div></div><br><div class="gmail_quote"></div><div class="gmail_quote"><div dir="ltr">On Fri, May 18, 2018 at 10:51 AM trent shipley <<a href="mailto:trent.shipley@gmail.com" target="_blank" rel="noreferrer">trent.shipley@gmail.com</a>> wrote:<br></div></div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">
<p class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-p1">The below produces an error. And I am very proud that I could use the GHCi debugging tools to get this far.</p><p class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-p1"><font face="monospace">merge [] []</font> works.</p><p class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-p1"><font face="monospace">merge [1] [] </font>works.</p><p class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-p1">I don't know why the failing example fails. It should return:</p><p class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-p1"><font face="monospace">[4,5]</font></p><p class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-p1">Help to unstuck is appreciated.</p><p class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-p1"><span class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-s1"><font face="monospace">:step merge [4,5] []</font></span></p>
<p class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-p1"><span class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-s1"><font face="monospace">*** Exception: ex6_8.hs:(12,1)-(16,66): Non-exhaustive patterns in function merge</font></span></p><p class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-p1">Given:</p><p class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-p1"><font face="monospace">merge :: Ord a => [a] -> [a] -> [a]</font></p><p class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-p1"><font face="monospace">merge [] [] = []</font></p><p class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-p1"><font face="monospace">merge [x] [] = [x]</font></p><p class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-p1"><font face="monospace">merge [] [y] = [y]</font></p><p class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-p1"><font face="monospace">merge first@(x:xs) second@(y:ys) | x <= y = x : merge xs second</font></p><p class="m_8890760677452331982m_8601678838629474277m_-7547731111719701566inbox-inbox-p1"><font face="monospace"> | otherwise = y : merge first ys</font></p><div><br></div></div></blockquote></div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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