<div dir="ltr"><div style="color:rgb(33,33,33);font-size:13px">Looks like you're missing the square brackets around your x in the definition of reverse (the ++ takes two lists, hence the error message)</div><div style="color:rgb(33,33,33);font-size:13px"><br></div><div style="color:rgb(33,33,33);font-size:13px">Try this</div><div style="color:rgb(33,33,33);font-size:13px"> </div><div style="color:rgb(33,33,33);font-size:13px">palindrome :: [t] -> [t]</div><div style="color:rgb(33,33,33);font-size:13px">palindrome xs = xs ++ (reverse xs)</div><div style="color:rgb(33,33,33);font-size:13px"> where</div><div style="color:rgb(33,33,33);font-size:13px"> reverse [] = []</div><div style="font-size:13px"><font color="#212121"> reverse (x:xs) = (reverse xs) ++ </font><b><font color="#0000ff">[</font></b><font color="#212121">x</font><b><font color="#0000ff">]</font></b></div><div><br></div><div><br></div>to declare the type you can define it outside or annotate it within you're code<div><br><div><div style="font-size:13px;color:rgb(33,33,33)">palindrome :: [t] -> [t]</div><div style="font-size:13px;color:rgb(33,33,33)">palindrome xs = xs ++ (reverse xs)</div><div style="font-size:13px;color:rgb(33,33,33)"> where</div><div style="font-size:13px"><div><font color="#212121"> </font><b><font color="#0000ff"> reverse :: [t] -> [t]</font></b></div></div><div style="font-size:13px;color:rgb(33,33,33)"> reverse [] = []</div><div style="font-size:13px"><font color="#212121"> reverse (x:xs) = (reverse xs) ++ </font><b><font color="#0000ff">[</font></b><font color="#212121">x</font><b><font color="#0000ff">]</font></b></div><br><div class="gmail_quote"><div dir="ltr">On Tue, Jul 4, 2017 at 12:07 PM Jona Ekenberg <<a href="mailto:saikyun@gmail.com">saikyun@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">Hello,<div><br></div><div>I'm currently going through the exercises in chapter 3 of Real World Haskell. One of the exercises challenges me to create a function which takes a list and makes a palindrome of it.</div><div><br></div><div>I tried to solve it this way:</div><div><div>palindrome :: [t] -> [t]</div><div>palindrome xs = xs ++ (reverse xs)</div><div> where</div><div> reverse [] = []</div><div> reverse (x:xs) = (reverse xs) ++ x</div></div><div><br></div><div>But when I try to compile this I get this error:</div><div><div>Kapitel3.hs:14:32-33: error: …</div><div> • Couldn't match type ‘t’ with ‘[t]’</div><div> ‘t’ is a rigid type variable bound by</div><div> the type signature for:</div><div> palindrome :: forall t. [t] -> [t]</div><div> at /Users/jona/programmering/haskell/boken/Kapitel3.hs:13:15</div><div> Expected type: [[t]]</div><div> Actual type: [t]</div><div> • In the first argument of ‘reverse’, namely ‘xs’</div><div> In the second argument of ‘(++)’, namely ‘(reverse xs)’</div><div> In the expression: xs ++ (reverse xs)</div><div> • Relevant bindings include</div><div> xs :: [t]</div><div> (bound at /Users/jona/programmering/haskell/boken/Kapitel3.hs:14:12)</div><div> palindrome :: [t] -> [t]</div><div> (bound at /Users/jona/programmering/haskell/boken/Kapitel3.hs:14:1)</div><div>Compilation failed.</div></div><div><br></div><div>It looks like I have used a function that want a list of lists, but I don't understand where.</div><div>Also, is there some way to declare the type of my reverse-function in this case?</div><div><br></div><div>Kind regards,</div><div>Jona Ekenberg</div></div>
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</blockquote></div></div></div></div><div dir="ltr">-- <br></div><div data-smartmail="gmail_signature"><div dir="ltr"><div><font color="#000000">Frederic Cogny<br></font></div><font color="#000000"><font>+33 </font><span>7 83 12 61 69</span></font><br></div></div>