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    <p>You <em>are</em> on the right path; lets take a look at some changes to your code I would suggest.</p> <p>Your <code>add1</code> function is quite close, but you've got two small problems - the first is the pattern match you provide: to be syntactically correct, you need the whole match to be inside the parentheses. The second is the second cons (colon). cons has a type of <code>a -&gt; [a] -&gt; [a]</code>, so it works perfectly with <code>(x+y)</code> as the first parameter; however, since <code>xs</code> has type <code>[Int]</code> already, you don't need to supply <code>: []</code>.</p> <pre><code>add1 (x:y:xs) = (x+y) : xs </code></pre> <p>Next, because you are dealing entirely with <code>Int</code>s and lists of <code>Int</code>s, using some type <code>a</code> does not make sense in this context.</p> <pre><code>push :: [Int] -&gt; Int -&gt; [Int] </code></pre> <p>Finally, your workhorse function. Because of the lack of loop constructs in Haskell, the way to do a user input loop (also known an <a href="http://en.wikipedia.org/wiki/REPL" rel="nofollow noreferrer">REPL</a>) is through recursion. Because of this, it would make some sense to accept a parameter. Let's make that your <code>[Int]</code> stack. The function to read one line from stdin as a string is <code>getLine</code>, which has type <code>IO String</code>. Finally, you actually have to handle that input. For simplicity, I've just included that logic in a <code>case</code> statement in <code>xcl</code>, but it could as well have been done using <code>dispatch</code> or a similar function (In fact, if your RPN calculater becomes more complicated, this would have its merits). The action for each case should recurse into your xcl "loop" with a modified stack. When you quit, it should just exit - which is a good use of <code>return</code>.</p> <pre><code>xcl :: [Int] -&gt; IO () xcl st = do print st answer &lt;- getLine case answer of "q" -&gt; return () "c" -&gt; xcl ([] ::[Int]) "+" -&gt; xcl $ add1 st x -&gt; xcl $ push st $ read x </code></pre> <p>In fact, you could take this one step further and protect against exceptions - what will happen when the user passes in some non-function, non-number string? The code above will fail, with a "no parse" exception. The best way around that is to use <code>reads</code> in place of <code>read</code>, as is discussed in <a href="https://stackoverflow.com/questions/5121371/how-to-catch-a-no-parse-exception-from-the-read-function-in-haskell/5121537#5121537">this answer</a>. <code>reads</code> returns either a list with a single entry - a tuple containing a parsed number and a remaining string, or an empty list (indicating a failed read). For example:</p> <pre><code> x -&gt; case (reads x) of [(num,_)] -&gt; xcl $ push st num _ -&gt; xcl st </code></pre>
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    1. COThank you! I got the program to work :)
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