Descriptive assertions using new F# 4.0 feature

*.fs

open FSharp.Quotations
open System.Diagnostics
open System

type Debug with
    [<Conditional("DEBUG")>]
    static member AssertThat([<ReflectedDefinition(true)>] condition: Expr<bool>) = 
        match condition with
        | Patterns.WithValue(value, t, expr) when t = typeof<bool> -> 
            let message = string expr //how do I get unparsed F# here?
            Debug.Assert(unbox value, message)
        | _ -> invalidArg "condition" (sprintf "Unexpected expressions: %A" condition)

Debug.AssertThat(42 = 42)
Debug.AssertThat(42 = 43)

something like this (compile with --quotations-debug+):

open FSharp.Quotations
open System.Diagnostics
open System

module P = Patterns
module DP = DerivedPatterns

type Debug with
    [<Conditional("DEBUG")>]
    static member AssertThat([<ReflectedDefinition(true)>] condition: Expr<bool>) = 
        match condition with
        | P.WithValue(value, t, expr) when t = typeof<bool> -> 
            let text = 
                match expr.CustomAttributes with
                | [P.NewTuple([DP.String("DebugRange"); P.NewTuple([DP.String(file); DP.Int32(startLine); DP.Int32(startCol); DP.Int32(endLine); DP.Int32(endCol)])])] ->
                    failwith "TODO: read file and extract text between startline\\startcol and endline\\endcol"
                | _ -> string expr
            Debug.Assert(unbox value, text)
        | _ -> invalidArg "condition" (sprintf "Unexpected expressions: %A" condition)

Debug.AssertThat(42 = 42)
Debug.AssertThat(42 = 43)```