BUG: walkAST() may give Error: Cannot walk expression. Unknown object type 'list'

[HenrikBengtsson]

In futureverse/future#156, it was reported that:

> y <- future::future_lapply(1L, function(a, b) typeof(b), b = list())
Error: Cannot walk expression. Unknown object type 'list'

with traceback:

1: future::future_lapply(1, function(a, b) typeof(b), b = list())
2: getGlobalsAndPackages(expr, envir = envir, tweak = tweakExpression, globals =
3: globalsOf(expr, envir = envir, substitute = FALSE, tweak = tweak, dotdotdot =
4: findGlobals(expr, envir = envir, ..., method = method, tweak = tweak, substit
5: tweak(expr)
6: walkAST(expr, call = tweakFutureAssignmentCall)
7: walkAST(expr[[cc]], atomic = atomic, name = name, call = call, pairlist = pai
8: stop("Cannot walk expression. Unknown object type ", sQuote(typeof(expr)), ca

In the above call, expr is effectively

> expr <- substitute(FUN(b = B), list(B = list()))
> str(expr)
 language FUN(b = list())

and walking the AST on that expression gives the error:

> globals::walkAST(expr)
Error: Cannot walk expression. Unknown object type 'list'

The error is triggered on the second component of expr, which is of type list:

> str(expr[[2]])
 list()
> typeof(expr[[2]])
[1] "list"

Note, this is not the same as:

> expr <- substitute(FUN(b = list()))
> str(expr)
 language FUN(b = list())

because there we have:

> str(expr[[2]])
 language list()
> typeof(expr[[2]])
[1] "language"

The current walkAST() code assumes the latter, but not the former.

/cc @ck37

[HenrikBengtsson]

Another problematic type is expression;

> y <- future::future_lapply(1L, function(a, b) typeof(b), b = expression())
Error: Cannot walk expression. Unknown object type 'expression'

[HenrikBengtsson]

This should have now been fixed in the develop branch;

remotes::install_github("HenrikBengtsson/globals@develop")

I've added package unit tests for it. I'm currently running reverse-dependency package tests (for everything on CRAN and Bioconductor), which will take some time. I'll reopen if I find something unexpected.