F# returning value from query expression

Question

I am just starting to learn F#, and impressed by the type inference I thought I would try a function that gets the first record from a table (using query expressions, Linq style):

let getfirst data = 
    let result = query { for n in data do take 1 }
    result |> Seq.head

This works, the type is IQueryable<'a> -> 'a.

But why doesn't this version work?

let getfirst2 data = 
    query { for n in data do head }

Shouldn't for n in data do head give a scalar 'a just like last time? Can someone explain why the second version doesn't work, and how to make it work without using Seq.head?

Answer 1

The reason is that the query builder has a somewhat hacky overloaded Run method for running queries, with the following overloads:

QueryBuilder.Run : Quotations.Expr<'t> -> 't
QueryBuilder.Run : Quotations.Expr<Linq.QuerySource<'t, IEnumerable>> -> seq<'t>
QueryBuilder.Run : Quotations.Expr<Linq.QuerySource<'t, IQueryable>> -> IQueryable<'t>

In your case, any of the overloads could apply, given a suitable type for data (though QuerySource<_,_> is a type which isn't ever meant to be used by user code, so two of the overloads are quite unlikely). Unfortunately, due to the strange way these overloads are defined (the first and second are actually extension methods defined in separate modules), the third one wins the overload resolution battle.