Error evaluating synthetic quotation expression from PowerPack - f#

Question

I am trying to figure out to modify quotations and then evaluate them. Here I am starting basic and just trying to create a quotation using the Quotations api. The quotation binds OK, but I get an error when evaluating.
#r #"FSharpPowerPack-2.0.0.0\bin\FSharp.PowerPack.dll"
#r #"FSharpPowerPack-2.0.0.0\bin\FSharp.PowerPack.Linq.dll"
open Microsoft.FSharp.Quotations
open Microsoft.FSharp.Linq.QuotationEvaluation
open Microsoft.FSharp.Linq
let hardway =
Expr.Let(
new Var("x", typeof<int>),
Expr.Value(10),
Expr.GlobalVar("x").Raw)
hardway.EvalUntyped()
Binding session to 'FSharp.PowerPack.Linq.dll'...
System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
at Microsoft.FSharp.Collections.MapTreeModule.find[TValue,a](IComparer`1 comparer, TValue k, MapTree`2 m)
at Microsoft.FSharp.Linq.QuotationEvaluation.ConvExpr(ConvEnv env, FSharpExpr inp) in d:\codeplex\fspowerpack\May2010\src\FSharp.PowerPack.Linq\Linq.fs:line 459
at Microsoft.FSharp.Linq.QuotationEvaluation.ConvExpr(ConvEnv env, FSharpExpr inp) in d:\codeplex\fspowerpack\May2010\src\FSharp.PowerPack.Linq\Linq.fs:line 704
at Microsoft.FSharp.Linq.QuotationEvaluation.ConvExpr(ConvEnv env, FSharpExpr inp) in d:\codeplex\fspowerpack\May2010\src\FSharp.PowerPack.Linq\Linq.fs:line 677
at Microsoft.FSharp.Linq.QuotationEvaluation.CompileImpl[a](a e, Boolean eraseEquality) in d:\codeplex\fspowerpack\May2010\src\FSharp.PowerPack.Linq\Linq.fs:line 837
at Microsoft.FSharp.Linq.QuotationEvaluation.Expr.EvalUntyped(FSharpExpr ) in d:\codeplex\fspowerpack\May2010\src\FSharp.PowerPack.Linq\Linq.fs:line 854
at <StartupCode$FSI_0009>.$FSI_0009.main#()
Stopped due to error

Answer 1

To get this working using global variables, you'd need to write it like this:
let hardway =
Expr.Let(
Var.Global("x", typeof<int>),
Expr.Value(10),
(Expr.GlobalVar<int>("x")) )
hardway.EvalUntyped()
Var.Global and Expr.Global use some shared global dictionary of variables that the F# quotations library uses to make it possible to get the same variable instance without explicitly passing Var values around (as in Stringer's solution).
However, I think that creating Var value only once and then keeping a reference to the object (and ussing the same object in the expression) leads to more readable code, so I'd prefer Stringer's solution.
A few points about my code:
You need to use Var.Global instead of new Var because the second option doesn't store the variable in the global dictionary.
You need to specify the type explicitly in Expr.GlobalVar - if you don't do that, F# will use obj and that's a different variable (they are indexed by name and type).

Answer 2

I don't know how to use GlobalVar so I let others answer on this. Here's a workaround in waiting for a better solution:
let hardway =
let v = new Var("x", typeof<int>)
Expr.Let(
v,
Expr.Value(10),
Expr.Var(v))
let res = hardway.EvalUntyped() // res is 10

Answer 3

Unquote has a custom reflection-based evaluation engine which allows you to evaluate synthetic quotations by passing in a variable environment rather than needing to make the variable binding part of the expression itself. So you could do the following:
open Swensen.Unquote
open Microsoft.FSharp.Quotations
let unquoteway = Expr.Var(Var("x", typeof<int>))
let environment = Map.ofList [("x", box 10)]
unquoteway.Eval(environment)
This is interesting because the environment you pass in is the very environment used for all variable bindings and resolutions throughout the expression evaluation so variable scoping rules are honored:
let unquoteway =
Expr.NewTuple(
[Expr.Var(new Var("x", typeof<int>))
Expr.Let(new Var("x", typeof<string>), Expr.Value("hello"), Expr.Var(new Var("x", typeof<string>)))])
let environment = Map.ofList [("x", box 10)]
unquoteway.Eval(environment)
//FSI output:
val unquoteway : Expr = NewTuple (x, Let (x, Value ("hello"), x))
val environment : Map<string,obj> = map [("x", 10)]
val it : obj = (10, "hello")