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    <p><strong>Note</strong>: using an F# compiler here; I don't have an OCaml compiler on this computer.</p> <p>The last line of your <code>subst</code> function has an error: It should be as follows:</p> <pre><code>| L l :: t -&gt; L (subst a b l) :: (subst a b t) </code></pre> <p>So the complete code would look like this:</p> <pre><code>type 'a Sexp = | S of 'a | L of 'a Sexp list let rec subst (a) (b) (lst : 'a Sexp list) = match lst with | [] -&gt; lst | S s :: t -&gt; if s = a then (S b) :: (subst a b t) else (S s) :: (subst a b t) | L l :: t -&gt; L (subst a b l) :: (subst a b t) let test () = let (lst : int Sexp list) = [S 1; L [S 2; L [S 3]; S 4]; S 5] let a = 2 let b = 3 subst a b lst </code></pre> <p>The output of <code>test()</code> is</p> <pre><code>[S 1; L [S 3; L [S 3]; S 4]; S 5] </code></pre> <p>The reason is that your function <code>subst</code> returns a <code>'a Sexp list</code>. If you omit the <code>L</code> constructor from the last line, then <code>subst a b l</code> is of type <code>'a Sexp list</code>, which you are attempting to cons with another list of type <code>'a Sexp list</code>. This does not work.</p> <p>Nor was this your intention, since you wanted to end up with an entity of type <code>'a Sexp list</code>, which means you must cons an element of type <code>'a Sexp</code> with a list of type <code>'a Sexp list</code>. By specifying the <code>L</code> constructor, you are creating an element of type <code>'a Sexp list</code>, which you can now cons with the rest of the list.</p>
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