I often use typed holes to define functions for which I know the interface and intend to implement later. When I run cabal build, it stops after the first module it encounters with typed holes, hiding type errors that may exist in other modules.
Is there any way of typechecking a project, and only failing for typed holes after the entire project has been built and typechecked?
@chi very politely told me to Read The Docs. There seems to be two ways of configuring typed holes:
The default behaviour, which makes typed holes a compile-time error. This causes compilation to stop after the first module encountered with typed holes, hiding both type errors and typed holes in other modules.
-fdefer-typed-holes which will issue a warning upon encountering a typed hole, and go on to compile the rest of the project. If no other errors are encountered, a binary is built, with the typed holes demoted to runtime errors. On the one hand, all holes show up in the compiler output, but on the other, it is less than ideal for these to allow the build to succeed.
There is, however, a slightly hacky combination of flags that gets (almost) the desired behaviour:
-fdefer-typed-holes -Werror=typed-holes
This typechecks each module in the project, stopping for any (non-hole) type errors. If there are none in a given module, build prints out all the typed holes in the project and goes on to typecheck the rest of the project. Build only succeeds if it encounters no type errors or typed holes.
It would be even nicer if we could get type errors and typed holes in the same output, but you can't have everything.