How to call C# method taking a parameter of type Expression<T> from F# ?

Question

I'm tryng to use the Brahma library from F# and i need to call a method that takes an Exprssion as parameter and I don't know how to do it.

here is the C# code i'd like to translate :

// Create the computation provider
var computationProvider = new ComputationProvider();

// Create a data-parallel array and fill it with data
var data = new DataParallelArray<float>(computationProvider, 
    new[] { 0f, 1f, 2f, 3f, 4f, 5f, 6f });

// Compile the query
CompiledQuery query = computationProvider.Compile<DataParallelArray<float>>
(
    d => from value in d
    select value * 2f
);

// Run the query on this data
IQueryable result = computationProvider.Run(query, data);

// Print out the results
foreach (float value in result)
        Console.WriteLine(result[i]);

// Get rid of all the stuff we created
computationProvider.Dispose();
data.Dispose();
result.Dispose();

Source

Answer 1

Glancing through the F# source directory[1], I see:

/// Convert F# quotations to LINQ expression trees.
/// A more polished LINQ-Quotation translator will be published
/// concert with later versions of LINQ.
let rec ConvExpr env (inp:Expr) = ...

1. .../FSharp-1.9.6.2/source/fsharp/FSharp.PowerPack.Linq/Linq.fs

High level information on F# quotations at [2,3].

1. http://msdn.microsoft.com/en-us/library/dd233212(VS.100).aspx
2. http://tomasp.net/blog/fsharp-quotation-samples.aspx

Answer 2

You need to use F# quotations, but these produce F# expression (Expr) objects, and the translation to CLR expressions is a bit fiddly.

Start by writing your expression as a F# quotation:

let expr = <@ fun (d : DataParallelArray<float>) -> d.ToString() @>  // real implementation omitted

Now you need something that will convert this to a CLR (LINQ) expression. I found this little helper method by a chap named Michael (can't find full name to give full credit, sorry):

open Microsoft.FSharp.Quotations
open Microsoft.FSharp.Linq.QuotationEvaluation
open System.Linq.Expressions

let ToLinq (exp : Expr<'a -> 'b>) =
    let linq = exp.ToLinqExpression()
    let call = linq :?> MethodCallExpression
    let lambda = call.Arguments.[0] :?> LambdaExpression
    Expression.Lambda<Func<'a, 'b>>(lambda.Body, lambda.Parameters) 

(Note this depends on the F# PowerPack, and the PowerPack ToLinqExpression method is considered experimental, so this may not be acceptable for production code.)

Now you just apply the ToLinq method to the F# expression and pass the result to the desired method:

let clrExpression = expr |> ToLinq
// pass clrExpression to Compile method

(This worked for me with a simplified test; you may need to provide some extra type hints, and I haven't tried to translate the sequence expression from your real code.)