Display help message upon UndefVarError(:help) using REPL.REPLCompletitions.UndefVarError_hint

[mkitti]

This is an alternate implementation of #41749 to display a useful help message when the user types "help" or "help()" into the REPL. Rather than creating a new type in InteractiveUtils, this PR modifies the REPL error display by expanding REPL.REPLCompletitions.UndefVarError_hint

In contrast to #41749, this allows the user to create a variable named help. Upon doing so, the help message will not be displayed.

julia> help
ERROR: UndefVarError: help not defined
  Welcome to Julia 1.8.0-DEV.288. The full manual is available at

  https://docs.julialang.org

  as well as many great tutorials and learning resources:

  https://julialang.org/learning/

  For help on a specific function or macro, type ? followed by its name, e.g. ?cos, or ?@time, and press enter. Type ; to enter shell mode, ] to enter package mode.

julia> help()
ERROR: UndefVarError: help not defined
  Welcome to Julia 1.8.0-DEV.288. The full manual is available at

  https://docs.julialang.org

  as well as many great tutorials and learning resources:

  https://julialang.org/learning/

  For help on a specific function or macro, type ? followed by its name, e.g. ?cos, or ?@time, and press enter. Type ; to enter shell mode, ] to enter package mode.
Stacktrace:
 [1] top-level scope
   @ REPL[5]:1

julia> help = 5
5

julia> help
5

This follows the suggestion of @vtjnash to intercept the UnderVarError for help and of @DilumAluthge to implement this functionality in the REPL code.

As shown above, this iteration still shows ERROR: UndefVarError: help not defined upon entering "help" and also shows a stacktrace for help(). A mechanism to hide the error would be nice, but is beyond the scope of this pull request.

[DilumAluthge]

Looks like this somehow broke make docs?

[KristofferC]

This new API looks quite a lot like the register_error_hint (

julia/base/experimental.jl

Line 168 in bdacfa2

Experimental.register_error_hint(handler, exceptiontype)

). Couldn't that be used?

In any case, adding a whole new public API for this seems excessive.

[mkitti]

That's the first I've heard of Experimental.register_error_hint. Sounds like that could address #17062 in general as well.

My inclination then is to go back to InteractiveUtils and register an error hint from there since having the help message displayed only makes sense when interactive.

[vtjnash]

The hook is also already registered for REPL and can be extended to add this text for handling "help". See #40545 :)

[KristofferC]

Just as a remark, Python seems to implement it in a similar way to #41749

>>> def f():
...     return help
... 
>>> x = f()
>>> type(x)
<class 'site._Helper'>
>>> x
Type help() for interactive help, or help(object) for help about object.

[mkitti]

Do we still want to show the error in this specific case of help? The implementation here and in #41749 preempts the error from even showing.

The suggestion hook seems to display after showing the error.

[mkitti]

I've force pushed a new commit that implements a much simpler change in REPL.REPLCompletitions.UndefVarError_hint. ERROR: UndefVarError: help not defined is now displayed which was not shown in the prior iteration or in the #41749 implementation.

We may want to add a mechanism to suppress the error message in the future, but that is for another pull request.

[mkitti]

Looks like all the tests pass this time.

[mkitti]

Perhaps I spoke too soon. I'm not sure what's happening with buildbot/package_freebsd64 build, but I'm pretty sure it is unrelated.

Any other thoughts or comments in the meantime?

[timholy]

Thanks @mkitti, this will be a very nice usability enhancement.