Composable FLinq expressions
- by Daniel
When doing linq-to-sql in c#, you could do something like this:
var data = context.MyTable.Where(x => x.Parameter > 10);
var q1 = data.Take(10);
var q2 = data.Take(3);
q1.ToArray();
q2.ToArray();
This would generate 2 separate SQL queries, one with TOP 10, and the other with TOP 3. In playing around with Flinq, I see that:
let data = query <@ seq { for i in context.MyTable do if x.Parameter > 10 then yield i } @>
data |> Seq.take 10 |> Seq.toList
data |> Seq.take 3 |> Seq.toList
is not doing the same thing. Here it seems to do one full query, and then do the "take" calls on the client side. An alternative that I see used is:
let q1 = query <@ for i in context.MyTable do if x.Param > 10 then yield i } |> Seq.take 10 @>
let q2 = query <@ for i in context.MyTable do if x.Param > 10 then yield i } |> Seq.take 3 @>
These 2 generate the SQL with the appropriate TOP N filter. My problem with this is that it doesn't seem composable. I'm basically having to duplicate the "where" clause, and potentially would have to duplicate other other subqueries that I might want to run on a base query. Is there a way to have F# give me something more composable?
(I originally posted this question to hubfs, where I have gotten a few answers, dealing with the fact that C# performs the query transformation "at the end", i.e. when the data is needed, where F# is doing that transformation eagerly.)
