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copied!<p>What you have doesn't sound to me like you need a heterogeneous list. Maybe you're looking for something like this?</p> <pre><code>data Foo = Foo Int data Bar = Bar Int data Arg = StringArg String | IntArg Int | DoubleArg Double data Tree = Group Arg Arg [Item] data Item = Item Foo Bar example :: Tree example = Group (StringArg "str") (IntArg 42) [Item (Foo 1) (Bar 2), Item (Foo 12) (Bar 36)] </code></pre> <p>Note that we could even create a list of <code>Arg</code>s of different "sub-types". For example, <code>[StringArg "hello", IntArg 3, DoubleArg 12.0]</code>. It would still be a homogeneous list, though.</p> <p>===== EDIT =====</p> <p>There are a few ways you could handle the "default argument" situation. Suppose the <code>Bar</code> argument in an item is optional. My first thought is that while it may be optional for the user to specify it, when I store the data I want to include the default argument. That way, determining a default is separated from the code that actually does something with it. So, if the user specifies a <code>Foo</code> of 3, but doesn't supply a <code>Bar</code>, and the default is <code>Bar 77</code>, then I create my item as:</p> <pre><code>Item (Foo 3) (Bar 77) </code></pre> <p>This has the advantage that functions that operate on this object don't need to worry about defaults; both parameters will always be present as far as they are concerned.</p> <p>However, if you really want to omit the default arguments in your data structure, you could do somthing like this:</p> <pre><code>data Bar = Bar Int | DefaultBar example = Group (StringArg "str") (IntArg 42) [Item (Foo 1) (Bar 2), Item (Foo 12) DefaultBar] </code></pre> <p>Or even:</p> <pre><code>data Item = Item Foo Bar | ItemWithDefaultBar Foo </code></pre> <p>===== Edit #2 =====</p> <p>So perhaps you could use something like this:</p> <pre><code>data ComplicatedItem = ComplicatedItem { location :: (Double, Double), size :: Int, rotation :: Double, . . . and so on . . . } defaultComplicatedItem = ComplicatedItem { location = (0.0,0.0), size = 1, rotation = 0.0), ... } </code></pre> <p>To create a <code>ComplicatedItem</code>, the user only has to specify the non-default parameters:</p> <pre><code>myComplicatedItem = defaultComplicatedItem { size=3 } </code></pre> <p>If you add new paramters to the <code>ComplicatedItem</code> type, you need to update <code>defaultComplicatedItem</code>, but the definition for <code>myComplicatedItem</code> doesn't change.</p> <p>You could also override the <code>show</code> function so that it omits the default parameters when printing.</p>

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