Hi Roger,<br><br><div class="gmail_quote">On 24 November 2011 10:43, Costello, Roger L. <span dir="ltr">&lt;<a href="mailto:costello@mitre.org">costello@mitre.org</a>&gt;</span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">

<div id=":y6">data Var = V | W | X | Y | Z<br>
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data Lambda = Variable Var | Application Lambda Lambda | Abstraction (Variable Var) Lambda</div></blockquote></div><div><br></div>Variable is a constructor of the data type Lambda. Abstraction is another. You use Variable as if it was a data type while defining Abstraction.<br>

<br><div>data Lambda = Variable Var | Application Lambda Lambda | Abstraction Lambda Lambda<br clear="all"><div><br></div><div>You can use the above declaration to successfully create your example lambda terms. However, it doesn&#39;t stop you from constructing things like:</div>

<div><br></div><div>Abstraction (Application .. ..) (Variable ..)</div><div><br></div><div>If you want to go to that land, you either need smart constructors (simpler) or GADTs (more hairy, unfortunately).</div><div><br>
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<div>HTH,</div><div>Ozgur</div>
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