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    <p>Two thoughts (wild guesses?) it looks as if it is failing to produce the right type signature in the <code>Just obj</code> clause and/or from <code>readObj name</code>, and it might be interesting to try something like this:</p> <pre><code>loader' :: [(String, ObjType)] -&gt; String -&gt; (ObjType, String -&gt; ObjType) loader' objs name = case findObj objs name of Nothing → (new_obj, loader' new_objs) where new_objs = [(name, new_obj)] ++ objs new_obj = readObj name Just obj → (obj, loader' objs) loader = loader' [] </code></pre> <p>I'm not saying that that will fix it (but I suppose that it might); rather, I'm saying that it may transform the problem into one that makes more sense, or otherwise shed light on the situation.</p> <p>Edit:</p> <p>Applying Tom Lokhorst's suggestion of naming the cache type leads us to this:</p> <pre><code>type Cache = [(String, ObjType)] loader' :: Cache -&gt; String -&gt; (ObjType, ????) loader' objs name = case findObj objs name of Nothing → (new_obj, loader' new_objs) where new_objs = [(name, new_obj)] ++ objs new_obj = readObj name Just obj → (obj, loader' objs) loader = loader' [] </code></pre> <p>which makes it obvious what the problem is. The type of the second result of loader' is a function that takes a string an produces a pair consisting of an ObjType and a function that takes a string an produces a pair consisting of an ObjType and a function that takes a string an produces a pair consisting of an ObjType and a function that takes a string an produces a pair consisting of an ObjType and ... you get the picture.</p> <p>If you rewrite it like this:</p> <pre><code>type Cache = [(String, ObjType)] loader' :: Cache -&gt; String -&gt; (ObjType, Cache) loader' objs name = case findObj objs name of Nothing → (new_obj, new_objs) where new_objs = [(name, new_obj)] ++ objs new_obj = readObj name Just obj → (obj, objs) </code></pre> <p>it should compile but you'll have to change how you use it.</p>
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