I've thought about this as well. Coming from the .NET world I'm quite
familiar with the Linq.Expression namespace. I too have wondered about
this. For those who are not familiar with how IQueryable<T> works in Linq,
it's something like the following (in clojure):
(def query (q (db :people)
(where #(> (:age %) 21))
:last-name
distinct))
The (db :people) call will return a IQueryable object. The q macro will
then take all the clauses in the query, wrap them up into an AST, and then
create a lazy seq. At this point in the code, nothing has been executed.
Upon calling (first query), the q macro hands the AST to the IQueryable
object, and then calls Execute on the object. Execute does whatever it
pleases with the AST, and then returns a lazy seq of the results.
A C# tutorial on this, is available here (
http://wekeroad.com/2007/07/02/linq-how-to-use-linq-to-query-just-about-anything/
)
I see this fitting into the Clojure ecosystem in a very interesting way:
Seq abstraction -
Lazy evaluation
Must run on the local CPU (for map, reduce, etc.)
Must run in order
Data provider can't influence execution
Reducers -
Data provider can influence execution
Must run on the local CPU
Can run in any order
IQueryable abstraction -
Data provider has full control over execution
Code can be run anywhere
Provider must compile code.
Example providers of IQueryable:
Entity Framework (Linq over SQL)
Linq to XML (Linq over XML)
Linq to GPU (run queries on a graphics processor)
http://brahma.ananthonline.net/
I've also seen index providers for in-memory data structures
So, I guess the original question still remains. Would Clojure benefit from
an AST abstraction? I know we have macros, but is that enough?
I'd love input on this topic.
Timothy Baldridge
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