Interactive Service: Embedding F# Interactive
This tutorial demonstrates how to embed F# interactive in your application. F# interactive is an interactive scripting environment that compiles F# code into highly efficient IL code and executes it on the fly. The F# interactive service allows you to embed F# evaluation in your application.
NOTE: There is a number of options for embedding F# Interactive. The easiest one is to use the
fsi.exe
process and communicate with it using standard input and standard output. In this tutorial, we look at calling F# Interactive directly through .NET API. However, if you have no control over the input, it is a good idea to run F# interactive in a separate process. One reason is that there is no way to handleStackOverflowException
and so a poorly written script can terminate the host process. Remember that while calling F# Interactive through .NET API,--shadowcopyreferences
option will be ignored. For detailed discussion, please take a look at this thread. NOTE: IfFsiEvaluationSession.Create
fails with an error saying thatFSharp.Core.dll
cannot be found, add theFSharp.Core.sigdata
andFSharp.Core.optdata
files. More info here.
However, the F# interactive service is still useful, because you might want to wrap it in your own executable that is then executed (and communicates with the rest of your application), or if you only need to execute a limited subset of F# code (e.g. generated by your own DSL).
Starting the F# interactive
First, we need to reference the libraries that contain the F# interactive service:
#r "FSharp.Compiler.Service.dll"
open FSharp.Compiler.Interactive.Shell
open FSharp.Compiler.Tokenization
To communicate with F# interactive, we need to create streams that represent input and output. We will use those later to read the output printed as a result of evaluating some F# code that prints:
open System
open System.IO
open System.Text
// Initialize output and input streams
let sbOut = new StringBuilder()
let sbErr = new StringBuilder()
let inStream = new StringReader("")
let outStream = new StringWriter(sbOut)
let errStream = new StringWriter(sbErr)
// Build command line arguments & start FSI session
let argv = [| "C:\\fsi.exe" |]
let allArgs =
Array.append argv [| "--noninteractive" |]
let fsiConfig =
FsiEvaluationSession.GetDefaultConfiguration()
let fsiSession =
FsiEvaluationSession.Create(fsiConfig, allArgs, inStream, outStream, errStream)
Evaluating and executing code
The F# interactive service exposes several methods that can be used for evaluation. The first
is EvalExpression
which evaluates an expression and returns its result. The result contains
the returned value (as obj
) and the statically inferred type of the value:
/// Evaluate expression & return the result
let evalExpression text =
match fsiSession.EvalExpression(text) with
| Some value -> printfn "%A" value.ReflectionValue
| None -> printfn "Got no result!"
This takes a string as an argument and evaluates (i.e. executes) it as F# code.
evalExpression "42+1" // prints '43'
This can be used in a strongly typed way as follows:
/// Evaluate expression & return the result, strongly typed
let evalExpressionTyped<'T> (text) =
match fsiSession.EvalExpression(text) with
| Some value -> value.ReflectionValue |> unbox<'T>
| None -> failwith "Got no result!"
evalExpressionTyped<int> "42+1" // gives '43'
The EvalInteraction
method can be used to evaluate side-effectful operations
such as printing, declarations, or other interactions that are not valid F# expressions, but can be entered in
the F# Interactive console. Such commands include #time "on"
(and other directives), open System
all declarations and other top-level statements. The code
does not require ;;
at the end. Just enter the code that you want to execute:
fsiSession.EvalInteraction "printfn \"bye\""
The EvalScript
method allows to evaluate a complete .fsx script.
File.WriteAllText("sample.fsx", "let twenty = 10 + 10")
fsiSession.EvalScript "sample.fsx"
Catching errors
EvalExpression
, EvalInteraction
and EvalScript
are awkward if the
code has type checking warnings or errors, or if evaluation fails with an exception.
In these cases you can use EvalExpressionNonThrowing
, EvalInteractionNonThrowing
and EvalScriptNonThrowing
. These return a tuple of a result and an array of FSharpDiagnostic
values.
These represent the errors and warnings. The result part is a Choice<_,_>
between an actual
result and an exception.
The result part of EvalExpression
and EvalExpressionNonThrowing
is an optional FSharpValue
.
If that value is not present then it just indicates that the expression didn't have a tangible
result that could be represented as a .NET object. This situation shouldn't actually
occur for any normal input expressions, and only for primitives used in libraries.
File.WriteAllText("sample.fsx", "let twenty = 'a' + 10.0")
let result, warnings =
fsiSession.EvalScriptNonThrowing "sample.fsx"
// show the result
match result with
| Choice1Of2 () -> printfn "checked and executed ok"
| Choice2Of2 exn -> printfn "execution exception: %s" exn.Message
Gives:
execution exception: Operation could not be completed due to earlier error
// show the errors and warnings
for w in warnings do
printfn "Warning %s at %d,%d" w.Message w.StartLine w.StartColumn
Gives:
Warning The type 'float' does not match the type 'char' at 1,19
Warning The type 'float' does not match the type 'char' at 1,17
For expressions:
let evalExpressionTyped2<'T> text =
let res, warnings =
fsiSession.EvalExpressionNonThrowing(text)
for w in warnings do
printfn "Warning %s at %d,%d" w.Message w.StartLine w.StartColumn
match res with
| Choice1Of2 (Some value) -> value.ReflectionValue |> unbox<'T>
| Choice1Of2 None -> failwith "null or no result"
| Choice2Of2 (exn: exn) -> failwith (sprintf "exception %s" exn.Message)
evalExpressionTyped2<int> "42+1" // gives '43'
Executing in parallel
By default the code passed to EvalExpression
is executed immediately. To execute in parallel, submit a computation that starts a task:
open System.Threading.Tasks
let sampleLongRunningExpr =
"""
async {
// The code of what you want to run
do System.Threading.Thread.Sleep 5000
return 10
}
|> Async.StartAsTask"""
let task1 =
evalExpressionTyped<Task<int>> (sampleLongRunningExpr)
let task2 =
evalExpressionTyped<Task<int>> (sampleLongRunningExpr)
Both computations have now started. You can now fetch the results:
task1.Result // gives the result after completion (up to 5 seconds)
task2.Result // gives the result after completion (up to 5 seconds)
Type checking in the evaluation context
Let's assume you have a situation where you would like to typecheck code in the context of the F# Interactive scripting session. For example, you first evaluate a declaration:
fsiSession.EvalInteraction "let xxx = 1 + 1"
Now you want to typecheck the partially complete code xxx + xx
let parseResults, checkResults, checkProjectResults =
fsiSession.ParseAndCheckInteraction("xxx + xx")
The parseResults
and checkResults
have types ParseFileResults
and CheckFileResults
explained in Editor. You can, for example, look at the type errors in the code:
checkResults.Diagnostics.Length // 1
The code is checked with respect to the logical type context available in the F# interactive session based on the declarations executed so far.
You can also request declaration list information, tooltip text and symbol resolution:
// get a tooltip
checkResults.GetToolTip(1, 2, "xxx + xx", [ "xxx" ], FSharpTokenTag.IDENT)
checkResults.GetSymbolUseAtLocation(1, 2, "xxx + xx", [ "xxx" ]) // symbol xxx
The 'fsi' object
If you want your scripting code to be able to access the 'fsi' object, you should pass in an implementation of this object explicitly. Normally the one from FSharp.Compiler.Interactive.Settings.dll is used.
let fsiConfig2 =
FsiEvaluationSession.GetDefaultConfiguration(fsiSession)
Collectible code generation
Evaluating code in using FsiEvaluationSession generates a .NET dynamic assembly and uses other resources.
You can make generated code collectible by passing collectible=true
. However, code will only
be collected if there are no outstanding object references involving types, for example
FsiValue
objects returned by EvalExpression
, and you must have disposed the FsiEvaluationSession
.
See also Restrictions on Collectible Assemblies.
The example below shows the creation of 200 evaluation sessions. Note that collectible=true
and
use session = ...
are both used.
If collectible code is working correctly, overall resource usage will not increase linearly as the evaluation progresses.
let collectionTest () =
for i in 1 .. 200 do
let defaultArgs =
[| "fsi.exe"
"--noninteractive"
"--nologo"
"--gui-" |]
use inStream = new StringReader("")
use outStream = new StringWriter()
use errStream = new StringWriter()
let fsiConfig =
FsiEvaluationSession.GetDefaultConfiguration()
use session =
FsiEvaluationSession.Create(fsiConfig, defaultArgs, inStream, outStream, errStream, collectible = true)
session.EvalInteraction(sprintf "type D = { v : int }")
let v =
session.EvalExpression(sprintf "{ v = 42 * %d }" i)
printfn "iteration %d, result = %A" i v.Value.ReflectionValue
// collectionTest() <-- run the test like this
namespace FSharp
--------------------
namespace Microsoft.FSharp
type StringBuilder = interface ISerializable new: unit -> unit + 5 overloads member Append: value: bool -> StringBuilder + 25 overloads member AppendFormat: provider: IFormatProvider * format: string * arg0: obj -> StringBuilder + 14 overloads member AppendJoin: separator: char * [<ParamArray>] values: obj array -> StringBuilder + 9 overloads member AppendLine: unit -> StringBuilder + 3 overloads member Clear: unit -> StringBuilder member CopyTo: sourceIndex: int * destination: char array * destinationIndex: int * count: int -> unit + 1 overload member EnsureCapacity: capacity: int -> int member Equals: span: ReadOnlySpan<char> -> bool + 1 overload ...
--------------------
StringBuilder() : StringBuilder
StringBuilder(capacity: int) : StringBuilder
StringBuilder(value: string) : StringBuilder
StringBuilder(capacity: int, maxCapacity: int) : StringBuilder
StringBuilder(value: string, capacity: int) : StringBuilder
StringBuilder(value: string, startIndex: int, length: int, capacity: int) : StringBuilder
type StringReader = inherit TextReader new: s: string -> unit member Close: unit -> unit member Peek: unit -> int member Read: unit -> int + 2 overloads member ReadAsync: buffer: char array * index: int * count: int -> Task<int> + 1 overload member ReadBlock: buffer: Span<char> -> int member ReadBlockAsync: buffer: char array * index: int * count: int -> Task<int> + 1 overload member ReadLine: unit -> string member ReadLineAsync: unit -> Task<string> + 1 overload ...
--------------------
StringReader(s: string) : StringReader
type StringWriter = inherit TextWriter new: unit -> unit + 3 overloads member Close: unit -> unit member FlushAsync: unit -> Task member GetStringBuilder: unit -> StringBuilder member ToString: unit -> string member Write: value: char -> unit + 4 overloads member WriteAsync: value: char -> Task + 4 overloads member WriteLine: buffer: ReadOnlySpan<char> -> unit + 1 overload member WriteLineAsync: value: char -> Task + 4 overloads ...
--------------------
StringWriter() : StringWriter
StringWriter(formatProvider: IFormatProvider) : StringWriter
StringWriter(sb: StringBuilder) : StringWriter
StringWriter(sb: StringBuilder, formatProvider: IFormatProvider) : StringWriter
<summary> Represents an F# Interactive evaluation session. </summary>
static member FsiEvaluationSession.GetDefaultConfiguration: fsiObj: obj -> FsiEvaluationSessionHostConfig
static member FsiEvaluationSession.GetDefaultConfiguration: fsiObj: obj * useFsiAuxLib: bool -> FsiEvaluationSessionHostConfig
Evaluate expression & return the result
member FsiEvaluationSession.EvalExpression: code: string * scriptFileName: string -> FsiValue option
<summary> The value, as an object </summary>
Evaluate expression & return the result, strongly typed
val int: value: 'T -> int (requires member op_Explicit)
--------------------
type int = int32
--------------------
type int<'Measure> = int
member FsiEvaluationSession.EvalInteraction: code: string * scriptFileName: string * ?cancellationToken: Threading.CancellationToken -> unit
File.WriteAllText(path: string, contents: string) : unit
File.WriteAllText(path: string, contents: ReadOnlySpan<char>, encoding: Encoding) : unit
File.WriteAllText(path: string, contents: string, encoding: Encoding) : unit
val exn: exn
--------------------
type exn = Exception
<summary> Gets the message for the diagnostic </summary>
<summary> Gets the start line for the diagnostic </summary>
<summary> Gets the start column for the diagnostic </summary>
member FsiEvaluationSession.EvalExpressionNonThrowing: code: string * scriptFileName: string -> Choice<FsiValue option,exn> * FSharp.Compiler.Diagnostics.FSharpDiagnostic array
type Task = interface IAsyncResult interface IDisposable new: action: Action -> unit + 7 overloads member ConfigureAwait: continueOnCapturedContext: bool -> ConfiguredTaskAwaitable + 1 overload member ContinueWith: continuationAction: Action<Task,obj> * state: obj -> Task + 19 overloads member Dispose: unit -> unit member GetAwaiter: unit -> TaskAwaiter member RunSynchronously: unit -> unit + 1 overload member Start: unit -> unit + 1 overload member Wait: unit -> unit + 5 overloads ...
--------------------
type Task<'TResult> = inherit Task new: ``function`` : Func<obj,'TResult> * state: obj -> unit + 7 overloads member ConfigureAwait: continueOnCapturedContext: bool -> ConfiguredTaskAwaitable<'TResult> + 1 overload member ContinueWith: continuationAction: Action<Task<'TResult>,obj> * state: obj -> Task + 19 overloads member GetAwaiter: unit -> TaskAwaiter<'TResult> member WaitAsync: cancellationToken: CancellationToken -> Task<'TResult> + 4 overloads member Result: 'TResult static member Factory: TaskFactory<'TResult>
--------------------
Task(action: Action) : Task
Task(action: Action, cancellationToken: Threading.CancellationToken) : Task
Task(action: Action, creationOptions: TaskCreationOptions) : Task
Task(action: Action<obj>, state: obj) : Task
Task(action: Action, cancellationToken: Threading.CancellationToken, creationOptions: TaskCreationOptions) : Task
Task(action: Action<obj>, state: obj, cancellationToken: Threading.CancellationToken) : Task
Task(action: Action<obj>, state: obj, creationOptions: TaskCreationOptions) : Task
Task(action: Action<obj>, state: obj, cancellationToken: Threading.CancellationToken, creationOptions: TaskCreationOptions) : Task
--------------------
Task(``function`` : Func<'TResult>) : Task<'TResult>
Task(``function`` : Func<obj,'TResult>, state: obj) : Task<'TResult>
Task(``function`` : Func<'TResult>, cancellationToken: Threading.CancellationToken) : Task<'TResult>
Task(``function`` : Func<'TResult>, creationOptions: TaskCreationOptions) : Task<'TResult>
Task(``function`` : Func<obj,'TResult>, state: obj, cancellationToken: Threading.CancellationToken) : Task<'TResult>
Task(``function`` : Func<obj,'TResult>, state: obj, creationOptions: TaskCreationOptions) : Task<'TResult>
Task(``function`` : Func<'TResult>, cancellationToken: Threading.CancellationToken, creationOptions: TaskCreationOptions) : Task<'TResult>
Task(``function`` : Func<obj,'TResult>, state: obj, cancellationToken: Threading.CancellationToken, creationOptions: TaskCreationOptions) : Task<'TResult>
<summary> The errors returned by parsing a source file. </summary>
<summary> Some of the values in the field FSharpTokenInfo.Tag </summary>
<summary> Indicates the token is an identifier (synonym for FSharpTokenTag.Identifier) </summary>