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<p class="MsoNormal">Ok so I have an assessment, first part is:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><b>Consider the following declaration for a parameterised type of lists in which<o:p></o:p></b></p>
<p class="MsoNormal"><b>Nil represents the empty list, and Cons x xs represent a non-empty list with<o:p></o:p></b></p>
<p class="MsoNormal"><b>first element x and remaining list of elements xs:<o:p></o:p></b></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><b>data List a = Nil | Cons a (List a)<o:p></o:p></b></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Am I right in reading it like: Declare a List type which can take any parameter and is constructed by Nil or Cons which itself takes anything and a List of anything?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Also it seems strange to define a List without [] or : like in the prelude but I assume it’s just for the purposes of the question. Is the (List a) used just to make the order of evaluation explicit or is it some weird thing with tuples?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Ok so the actual question is:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><b>Q) Define a recursive function append :: List a -> List a -> List a that<o:p></o:p></b></p>
<p class="MsoNormal"><b>appends two lists together to give a single list.<o:p></o:p></b></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">So I’d have to do something similar to how append is done in the standard prelude I.e.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">(++) :: [a] -> [a] -> [a]<o:p></o:p></p>
<p class="MsoNormal">[] ++ ys = ys<o:p></o:p></p>
<p class="MsoNormal">(x:xs) ++ ys = x : (xs ++ ys)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">How do I do pattern matching on a user defined type? <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I was thinking something like:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><b>Nil append List a = List a<o:p></o:p></b></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">For the first line but the second line is confusing me. I’ve read some stuff on pattern matching for data constructors but I don’t know how relevant it is here.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Maybe something like this would work:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><b>Cons a (List a) append List b = a : (List a append List b)<o:p></o:p></b></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">But I’m getting ‘Data constructor not in scope’ for everything to do with List...it thinks it’s a data constructor although I’ve defined it as a type above.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Any help would be great <span style="font-family:"Segoe UI Emoji",sans-serif">
😊</span><o:p></o:p></p>
<p class="MsoNormal">Sent from <a href="https://go.microsoft.com/fwlink/?LinkId=550986">
Mail</a> for Windows 10<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Sent from <a href="https://go.microsoft.com/fwlink/?LinkId=550986">
Mail</a> for Windows 10</p>
<p class="MsoNormal"><o:p> </o:p></p>
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