Congratulations, graduate — you've already proven you know F#. You filled in the blanks, matched the patterns, piped the lists. But the Koans workflow was also kind of... slow? dotnet watch run, squint at terminal output, scroll to find which koan broke, fix it, wait 3 seconds, repeat. SageFs is the upgrade: same F# you learned, but with instant inline feedback, live test gutter markers, and no more terminal-squinting.
Pain you're leaving behind: The 2-4 second dotnet watch run cycle, terminal-only pass/fail output, the custom [<Koan>] framework that doesn't work anywhere else, and the gap between "I finished the exercises" and "I can build real things."
What you'll love immediately:
- Alt+Enter on any expression → result appears inline, not in the terminal
- Expecto tests with live gutter markers (✓/✗) — the natural evolution of koan assertions
- No more
dotnet runcycles — feedback in ~200ms - Your koan-learned skills (DUs, pipelines, options, pattern matching) applied to real domains
→ Start here: samples/from-koans/00-about-sagefs-koans.fsx (the roadmap — then work through 01-about-asserts.fsx → 21-about-filtering.fsx at your own pace)
// Koans taught you this:
// let actual_value = __
// AssertEquality expected_value actual_value
// dotnet run → FAIL → fix → run → PASS → next (~3 sec cycle)
// SageFs: just evaluate it.
let x = 1 + 1 // Alt+Enter → 2, right here, in ~200ms
// Your DU skills, applied to a real domain:
type OrderStatus =
| Pending | Shipped of tracking: string | Cancelled of reason: string
let describe = function
| Pending -> "⏳ Awaiting shipment"
| Shipped t -> $"📦 {t}"
| Cancelled r -> $"❌ {r}"
// Alt+Enter → instant result. No test framework needed for exploration.