Issue #11868 has been updated by Joe Rafaniello.


I agree, it seems the best place to put this information is in the SyntaxError exception.   If we need to keep compatibility by keeping the output to $stderr, I'm OK with that but I want to be able to rescue the SyntaxError and get the "file", "lineno" and the problem code (hint).
The hint or problem code could even be reconstructed exactly as Tsuyoshi Sawada says if we know the column position of the syntax error.  

Tools like rubocop would most likely want exactly what is currently printed to $stderr but as methods on some object such as the exception.

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Feature #11868: Proposal for RubyVM::InstructionSequence.compile to return an object containing the syntax error information currently written to STDERR
https://bugs.ruby-lang.org/issues/11868#change-56845

* Author: Joe Rafaniello
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
Currently, RubyVM::InstructionSequence.compile or RubyVM::InstructionSequence.new return a new InstructionSequence for valid ruby.

For invalid syntax, a SyntaxError is raised with a message of 'compile error'.  Meanwhile, the useful information, line number(s) and hint(s) to the invalid syntax location, is printed on standard error.  I am proposing this information be returned as an object in the event of a SyntaxError.

For example, here's good syntax:

~~~
RubyVM::InstructionSequence.new("x =1")
# => <RubyVM::InstructionSequence:<compiled>@<compiled>>
~~~


Here's bad syntax:

~~~
RubyVM::InstructionSequence.new("puts 'hi'\n puts 'hi2'\n\nthis.is -> not -> valid $ruby:syntax")

# => SyntaxError: compile error

# The useful hint and line number(s) are on standard error:
<compiled>:4: syntax error, unexpected keyword_not, expecting keyword_do_LAMBDA or tLAMBEG
this.is -> not -> valid $ruby:syntax
              ^
<compiled>:4: syntax error, unexpected tGVAR, expecting keyword_do_LAMBDA or tLAMBEG
this.is -> not -> valid $ruby:syntax
                             ^
~~~

Some ideas:

1. Add methods to all SyntaxError exceptions to get all parse failures.  For example: `syntax_error.parse_failures.each {|f| puts f.lineno; puts f.hint }`.  In the above example, it failed on line 4 twice and we see two "hints."

2. Create a new method to RubyVM::InstructionSequence to check ruby syntax that would allow us to see if the syntax is valid and if not,  the lineno and 'hint' for each parse failure.

Use case:  Rubocop[a] and other utilities[b] are really complicated and check for valid ruby syntax by creating a process to run ruby -wc with the script.
[a] https://github.com/bbatsov/rubocop/blob/86e1acf67794bf6dd5d65812b91df475e44fa320/spec/support/mri_syntax_checker.rb#L51-L63
[b] https://github.com/ManageIQ/manageiq/blob/6725fe52222c07d576a18126d2ff825ddc6dffd0/gems/pending/util/miq-syntax-checker.rb#L8-L13

It would be nice to remove all of this complexity and use RubyVM::InstructionSequence, which already has the information we need but in a more user friendly format.

Thanks!
Joe Rafaniello



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