Simple is better than complex

Sukera · December 19, 2018, 6:19am

Please search this forum and the issues on github - there has been A LOT of discussion about this, for example these threads

github.com/JuliaLang/julia

Global variable scope rules lead to unintuitive behavior at the REPL/notebook

opened 03:36AM - 21 Aug 18 UTC

closed 08:33PM - 28 Jan 20 UTC

IainNZ

REPL minor change

### Example 1 This came up with a student who upgraded from 0.6 to 1.0 direct…ly, so never even got a chance to see a deprecation warning, let alone find an explanation for new behavior: ```julia julia> beforefor = true true julia> for i in 1:2 beforefor = false end julia> beforefor # this is surprising bit true julia> beforeif = true true julia> if 1 == 1 beforeif = false end false julia> beforeif # Another surprise! false julia> function foo() infunc = true for i in 1:10 infunc = false end @show infunc end foo (generic function with 1 method) julia> foo() # "I don't get this" infunc = false ``` ### Example 2 ```julia julia> total_lines = 0 0 julia> list_of_files = ["a", "b", "c"] 3-element Array{String,1}: "a" "b" "c" julia> for file in list_of_files # fake read file lines_in_file = 5 total_lines += lines_in_file end ERROR: UndefVarError: total_lines not defined Stacktrace: [1] top-level scope at ./REPL[3]:4 [inlined] [2] top-level scope at ./none:0 julia> total_lines # This crushs the students willingness to learn 0 ``` I "get" why this happens in the sense that I think I can explain, with sufficient reference to the arcana in the manual about what introduces scopes and what doesn't, but I think that this is problematic for interactive use. In example one, you get a silent failure. In example two, you get an error message that is very there-is-no-spoon. Thats roughly comparable to some Python code I wrote in a notebook at work today. I'm not sure what the rules are in Python, but I do know that generally you can't assign to things at the global scope without invoking global. But at the REPL it does work, presumably because at the REPL the rules are different or the same logic as if they were all are in the scope of function is applied. I can't language-lawyer the rules enough to propose the concrete change I would like, and based on Slack this isn't even necessarily perceived as an issue by some people, so I don't know where to go with this except to flag it. Cross-refs: #19324 https://discourse.julialang.org/t/repl-and-for-loops-scope-behavior-change/13514 https://stackoverflow.com/questions/51930537/scope-of-variables-in-julia

And plenty of others.