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    <p>What your asking for is to be able to structurally define a type as the <em>difference</em> of two other types (the original relation and the projection definition). I honestly can't think of any language which would allow you to do that. Types can be structurally cumulative (<code>A with B</code>) since <code>A with B</code> is a structural sub-type of both <code>A</code> and <code>B</code>. However, if you think about it, a type operation <code>A less B</code> would actually be a <em>supertype</em> of <code>A</code>, rather than a sub-type. You're asking for an arbitrary, contravariant typing relation on naturally covariant types. It hasn't even been proven that sort of thing is sound with nominal existential types, much less structural declaration-point types.</p> <p>I've worked on this sort of modeling before, and the route I took was to constraint projections to one of three domains: <code>P</code> == <code>T</code>, <code>P</code> == <code>{F} where F in T</code>, <code>P</code> == <code>{$_1} where $_1 anonymous</code>. The first is where the projection is equivalent to the input type, meaning it is a no-op (<code>SELECT *</code>). The second is saying that the projection is a single field contained within the input type. The third is the tricky one. It is saying that you are allowing the declaration of some anonymous type <code>$_1</code> which has no <em>static</em> relationship to the input type. Presumably it will consist of fields which delegate to the input type, but we can't enforce that. This is roughly the strategy that LINQ takes.</p> <p>Sorry I couldn't be more helpful. I wish it were possible to do what you're asking, it would open up a lot of very neat possibilities.</p>
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    1. POLanguage features to implement relational algebra
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    1. USDaniel Spiewak
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    1. COI would love for you to provide me with some keys do decipher what you wrote in the second paragraph..
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      1. USJohn Nilsson
 

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