Warnings for monadic values not used
I would like two warnings. The first -fwarn-unused-monad-bind:
do doesFileExist "foo" ; return 1doesFileExist returns a value of IO Bool, not IO (), and does not bind it's result. This warning should be applied to both do notation and >> and should fire on all values which aren't m ().
The second -fwarn-wrong-bind:
do return (doesFileExist "foo") ; return 1doesFileExist returns a value of type IO (IO a), which is not bound. This warning should fire on all m (m a) values and also in >>.
The names of the flags are intended to be placeholders for a sensibly chosen name. I would make -fwarn-wrong-bind on as a default warning, and both turned on under -Wall. I'm not sure anyone has ever written the second construction without it being a serious error. In the case of IO it's really easy to miss that some side effects are no longer happening after a refactoring. I've just made this very mistake in fairly public way.
It seems that problems such as mapM space leaks are common, and that it is not widely known that such constructions without a named binding are always wrong. I've had to explain the problem to fairly experienced Haskell programmers.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 6.10.2 |
| Type | FeatureRequest |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | ndmitchell@gmail.com |
| Operating system | |
| Architecture |