From 58aa302f49311475cce11dcbd477df217e790ea6 Mon Sep 17 00:00:00 2001
From: Joel Stålnacke <joel@saker.fi>
Date: Fri, 13 Feb 2026 20:48:51 +0200
Subject: Initial commit

---
 examples/Basic/Program.fs | 40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)
 create mode 100644 examples/Basic/Program.fs

(limited to 'examples/Basic/Program.fs')

diff --git a/examples/Basic/Program.fs b/examples/Basic/Program.fs
new file mode 100644
index 0000000..a84f96e
--- /dev/null
+++ b/examples/Basic/Program.fs
@@ -0,0 +1,40 @@
+open FunctionalObjects
+
+// A record is like an interface
+type Printer = {
+    PrintMessage : string -> unit
+}
+
+// Let's define a printHelloWorld function which uses the Printer interface.
+//
+// Notice that the type annotation is not required because Printer is a record
+// and F# can infer the type of the variable based on the PrintMessage field.
+//
+// Because this pattern uses a concrete type for the interface, no runtime
+// reflection is needed and type checking happens completely at compile time.
+let printHelloWorld (printer : Printer) =
+    printer.PrintMessage "Hello World"
+
+// An instance of the record is a class aka an implementation of the interface
+let basePrinter : Printer =
+    { PrintMessage = fun s -> printfn "Message: %s" s }
+
+printfn "Using the base printer:"
+printHelloWorld basePrinter
+
+// We can create a sub-class of the base printer using Object.derive
+// 
+// Object.derive is simply 
+// let derive f o = f o
+// so it's completely syntactic sugar
+let screamingPrinter : Printer =
+    basePrinter
+    |> Object.derive (fun b ->
+        { b with
+            // We choose which methods to override
+            PrintMessage =
+                fun s -> b.PrintMessage (s.ToUpper()) })
+
+// printHelloWorld doesn't have to change when we switch the Printer.
+printfn "Using the screaming printer:"
+printHelloWorld screamingPrinter
-- 
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