When attempting to update a read-only var return an error struct rather
than a simple string (i.e., a Go `error` type). This makes it possible
to include the var name in the error message. This builds on commit
a33ecb2d that highlights the offending var name in the stack trace but
does not include the var name in the error message. With this change the
error message includes the offending var name.
Related #255
- Move NewEnvListVar to pkg/eval/vars.
- De-export GlobPattern, GlobFlag and ExternalCmd.
- Merge editor.go and chdir.go into eval.go, value_helper.go into compile_value.go.
- Remove eval_internal_test.go and replace it with a new test for $pid.
As with commit #eb2a792 I acknowledge that `golint` recommendations are
controversial and should not automatically be acted on. Nonetheless,
this change fixes legitimate lint issues such as copy/paste cleanups,
style problems I introduced (`i += 1` versus `i++`) or method/var comments
that are not in the preferred form.
I stumbled across a comment that began with "XXX". It was clearly meant as
a "TODO" comment. This changes all such occurrences. However, a few "XXX"
comments are ambiguous and a better prefix might be "WARNING". The "TODO"
prefix at least ensures someone, eventually, looks into the situation
and either rewords the comment or fixes the problem. This change means
everyone can assume searching for "// TODO" will find all such comments
rather than requiring they also know to search for "// XXX".
