Fsi

F# Interactive

Trigger On

• the task asks for F# Interactive, FSI, .fsx, or dotnet fsi
• a quick typed experiment is needed before changing compiled project code
• a script should reference NuGet packages directly with #r "nuget: ..."
• an investigation needs quick access to F# type inference, pattern matching, or pipelines
• a repeatable one-file probe is better than a temporary project

Do Not Use For

• long-lived application code that needs a compiled .fsproj
• production automation that should be versioned as a CLI, test project, or build target
• C# script work
• package restore from untrusted sources

Quick Start

Run an interactive REPL:

dotnet fsi

Run a script:

dotnet fsi scripts/check.fsx

Create a Unix executable script when that fits the repo:

#!/usr/bin/env -S dotnet fsi

printfn "Hello from FSI"

chmod +x scripts/check.fsx
./scripts/check.fsx

On Windows, run scripts with dotnet fsi scripts/check.fsx.

Workflow

1. Decide whether the request is a disposable REPL probe, a repeatable .fsx script, or code that should be promoted to a compiled F# project.
2. Put repeatable work in an .fsx file immediately. Add all required #r, #load, open, input path, and package source directives to the script instead of relying on hidden REPL state.
3. Keep package references pinned when the script should be reproducible, and use only trusted NuGet feeds or local feeds derived from __SOURCE_DIRECTORY__.
4. Run the script with dotnet fsi from a clean shell, passing the same arguments the user or CI will use.
5. Promote the script to an .fsproj when it needs tests, distribution, project references, or long-term CI coverage.

Current Upstream Notes

• The September 2026 F# Interactive reference keeps dotnet fsi as the supported command-line entry point for interactive sessions and .fsx scripts; it does not turn hidden REPL state into a reproducible workflow.
• Use repeatable .fsx files with explicit #r "nuget: ..." and #load directives once an experiment affects a repository task; do not rely on hidden REPL state.

Interactive Session Rules

• End REPL submissions with ;;.
• Multi-line input is allowed; FSI evaluates when it receives ;;.
• Previously evaluated values stay in the session, so do not rely on hidden session state in scripts.
• Use .fsx files for repeatability once an experiment matters.

let square x = x * x;;

[ 1 .. 5 ] |> List.map square;;

Script Patterns

Read And Summarize Text

Use ordinary .NET APIs directly from F# scripts.

open System
open System.IO

let summarize path =
    File.ReadLines path
    |> Seq.filter (String.IsNullOrWhiteSpace >> not)
    |> Seq.countBy (fun line -> line.Split(' ')[0])
    |> Seq.sortByDescending snd
    |> Seq.truncate 10
    |> Seq.toList

for key, count in summarize "input.log" do
    printfn $"{key}: {count}"

Write A Checked Output File

Keep script outputs deterministic and fail early when required inputs are missing.

open System
open System.IO

let input = "data/items.txt"
let output = "artifacts/items.normalized.txt"

if not (File.Exists input) then
    failwith $"Missing input file: {input}"

Directory.CreateDirectory(Path.GetDirectoryName output) |> ignore

File.ReadLines input
|> Seq.map (fun line -> line.Trim())
|> Seq.filter (String.IsNullOrWhiteSpace >> not)
|> Seq.distinct
|> Seq.sort
|> fun lines -> File.WriteAllLines(output, lines)

Reference NuGet Packages

Pin package versions for repeatable scripts. Only use package sources the repo trusts.

#r "nuget: Newtonsoft.Json, 13.0.3"

open Newtonsoft.Json

let payload = {| Name = "Ada"; Kind = "sample" |}
let json = JsonConvert.SerializeObject(payload)

printfn $"{json}"

FSI does not use referenced package build targets during restore by default. Enable them only when a trusted package requires them; pin the version and test from a fresh process:

#r "nuget: FSharp.Data, 6.6.0, usepackagetargets=true"

Keep usepackagetargets=false or omit the option for ordinary references. This controls package restore behavior; it does not turn an .fsx script into a compiled project.

Use #i only when an additional feed is required. Local feed paths must be absolute; construct them from __SOURCE_DIRECTORY__ instead of committing personal paths.

let localFeed =
    System.IO.Path.Combine(__SOURCE_DIRECTORY__, "../artifacts/packages")
    |> System.IO.Path.GetFullPath

#i $"nuget: {localFeed}"

Split Scripts With Load

#load evaluates another script and exposes it through the generated module name.

// MathHelpers.fsx
let square x = x * x

// Check.fsx
#load "MathHelpers.fsx"
open MathHelpers

printfn $"%d{square 12}"

Promote To A Project

Move from FSI to a compiled project when:

• the script has multiple dependencies, tests, or distribution needs
• startup time or restore behavior matters
• C# or other .NET callers need a stable assembly
• the code needs CI coverage beyond a smoke run

Start with:

dotnet new console -lang "F#" -o tools/Probe
dotnet build tools/Probe/Probe.fsproj

Validate

Use the simplest command that proves the script still runs:

dotnet fsi scripts/check.fsx
dotnet fsi scripts/check.fsx -- arg1 arg2

For scripts that reference packages, run from a clean shell at least once so hidden REPL state cannot mask missing #r, #load, or open directives.

Sources

• https://learn.microsoft.com/dotnet/fsharp/tools/fsharp-interactive/
• https://learn.microsoft.com/dotnet/fsharp/get-started/get-started-vscode#explore-f-with-scripts-and-the-repl
• https://learn.microsoft.com/dotnet/fsharp/language-reference/fsharp-interactive-options