Constant string expressions for type provider parameters

[NickDarvey]

Type providers like FSharp.Data.Npgsql accept a SQL string constant as a type argument.

e.g.

use cmd = DvdRental.CreateCommand<"SELECT title FROM public.film WHERE length > @longer_than">(dvdRental)

Can you think of a trick where this extension could reliably detect these strings? Nothing I've considered seems like good (or even possible) options:

1. Extending this extension to allow configurable regex so users could define CreateCommand< as a match for SQL.
2. Figuring a way to define an identity function named sql that can be used inside const().

[alfonsogarciacaro]

Probably with a comment, I was trying to avoid it because I found the function approach to be terser and because inline comments are not common in F#, but we'll likely need them for cases like this.

use cmd = DvdRental.CreateCommand<(* sql *) "SELECT title FROM public.film WHERE length > @longer_than">(dvdRental)