moonbitlang
"The error type of this function is never used" should be a warning rather than an Error
Bug Report
Environment
OS
Operating System: macOS
MoonBit CLI Tools Version
moon 0.1.20250918 (e5fa392 2025-09-18) ~/.moon/bin/moon moonc v0.6.28+d96d14d39 (2025-09-18) ~/.moon/bin/moonc moonrun 0.1.20250918 (e5fa392 2025-09-18) ~/.moon/bin/moonrun moon-pilot 0.0.1-850379f (2025-09-19) ~/.moon/bin/moon-pilot
Steps to Reproduce
Create a function with a return type of 'Unit raise Error'
Expected Behavior
See a compiler warning not an error. Because users may want to use that error in the future.
Actual Behavior
Cannot compile successfully.
This is actually intended behavior. By making unused error an error instead of a warning, users are enforced to think carefully whether a function may raise error, this helps avoid unnecessary error annotation and make code more robust. Do you have any use case where this behavior result in problems that cannot be fixed by simply removing the unnecessary error annotation?
Excessive compiler errors increase the cost of prototyping and experimentation. Suppose I have an unimplemented function A that, in its eventual full implementation, will throw a specific error. Today, however, the compiler forces me either to remove that error type or to hack around it with something like if (true) { raise Error }.
Allowing an incomplete implementation helps me first validate the main path. At the same time, keeping the intended error type lets downstream code prepare proper handling early and keeps both sides’ interfaces aligned—so we don’t have to retrofit the error type later and make every downstream caller change again.
