[bug]: `FlatMap` implementation in `fn` package is inverted

[gijswijs]

Pre-Submission Checklist

• I have searched the existing issues and believe this is a new bug.
• I am not asking a question about how to use lnd, but reporting a bug (otherwise open a discussion).

LND Version

v0.20.0 and all versions before with the fn package

LND Configuration

--

Backend Version

na

Backend Configuration

na

OS/Distribution

na

Bug Details & Steps to Reproduce

The implementation of the FlatMap method for the Result type is inverted:

lnd/fn/result.go

Lines 148 to 156 in 85a5bf2

	// FlatMap applies a kleisli endomorphic function that returns a Result to the
	// success value if it exists.
	func (r Result[T]) FlatMap(f func(T) Result[T]) Result[T] {
	if r.IsOk() {
	return r
	}
	return f(r.left)
	}

For a Result[T], FlatMap should apply f when the result is Ok, and propagate the error unchanged when it’s Err. The current code returns r on Ok and tries to use r.left when Err, which is wrong.

Expected Behavior

na

Debug Information

No response

Environment

No response

[gijswijs]

I'll fix this myself tomorrow, just created this issue for tracking.