Unexpected behavior of #==#

General Usage
1

Hi,
I was playing around with multi line comments (in Julia v0.5.1) , and got some interesting results.
So I tried:

julia> my#=comment=#var = 0

  • (generic function with 1 method)

And now I get:
julia> 3 * 4
0

So I just overloaded (*) for (Any,Any). That’s weird.
So, I wanted to retry in a way, that I can edit *, so:

import Base.*
my#=comment=#var = 0

This creates a new method for (*), so “hi” * false returns now 0.

When I try this in Jupyter via Juliabox (Version 0.5), I get an error, if I don’t include Base.* first, but than it also adds a new method to it.

So, does anybod know, why this happens?

Here’s my versioninfo():
Julia Version 0.5.1
Commit 6445c82* (2017-03-05 13:25 UTC)
Platform Info:
OS: Linux (x86_64-pc-linux-gnu)
CPU: AMD Phenom™ II X6 1100T Processor
WORD_SIZE: 64
BLAS: libblas
LAPACK: liblapack
LIBM: libm
LLVM: libLLVM-3.9.1 (ORCJIT, amdfam10)

1 Like
2

It’s parsed as juxtapose, i.e. multiplication.

3

That seems like it is probably unexpected behavior, which probably should be changed in the parser to not count as juxtaposition.

4

Thank you vary much for the answer, I didn’t think of that.

I also didn’t know you can overload operators that simply.

Also a thing that surprised me, though understandable:

If I open an new julia instance and run a+b=0, it just sets this as the only method for (+). But if use (+) once before in that instance, than I can’t redefine it like that, without including Base.+, and even than it would only add a new method, not “remove” the old ones.

5

In my opinion, this is a flaw in Julia. It is too easy to accidentally redefine a built-in operator. Indeed, I posted an issue last year on github to gripe about my experience when I made a small typo in a function – I put an = instead of a colon somewhere, or something similar (inside an array subscript!). The consequence was that “+” was redefined for the whole function, causing the function to fail with a mysterious error on a line that was distant from the line with the typo.

I would say that redefining a built-in operator should require that the programmer write a highly obvious chunk of code, or else a syntax error should result.

6

On the other hand, if there’s ever an international obfuscated julia code contest, this will be a useful feature to have. :wink:

julia> import Base.-

julia> my_var = 0;

julia> my-var = my_var + 1;

julia> 4 - 2
1
2 Likes
7

It occurred to me as well. The problem is that this happens even without importing the function:

julia> a-b=rand(15)
- (generic function with 1 method)

julia> rand(5) .- rand(5)
5-element Array{Array{Float64,1},1}:
 [0.83553, 0.846525, 0.447319, 0.829272, 0.14231, 0.671123, 0.532965, 0.704368, 0.280074, 0.180665, 0.833187, 0.589239, 0.335998, 0.216206, 0.992392]       
 [0.269979, 0.313953, 0.313245, 0.969569, 0.772667, 0.934492, 0.93995, 0.188244, 0.101232, 0.0525931, 0.12574, 0.692653, 0.471061, 0.366899, 0.462796]      
 [0.334985, 0.12022, 0.0730848, 0.65934, 0.588324, 0.160928, 0.274765, 0.267733, 0.730062, 0.253658, 0.837493, 0.00747138, 0.983055, 0.290284, 0.27298]     
 [0.000187563, 0.605232, 0.0321455, 0.212838, 0.103809, 0.557014, 0.625323, 0.266018, 0.363476, 0.591861, 0.544453, 0.17569, 0.769616, 0.00455865, 0.395421]
 [0.781084, 0.348457, 0.523047, 0.727615, 0.375016, 0.674879, 0.328631, 0.942764, 0.827079, 0.860394, 0.845891, 0.416686, 0.125326, 0.723758, 0.603509]

I expected to get a 5-element array of scalars, not an array of arrays, it took me a while to understand what was going on.

Actually, I can swear that when I defined the variable I didn’t even get in the REPL the message about (re-)definition of the method, but I couldn’t reproduce the issue afterwards (I should have taken a screenshot :frowning: )

8

Ref Infix operator definition syntax needs documentation · Issue #15483 · JuliaLang/julia · GitHub, https://github.com/JuliaLang/julia/issues/16383

1 Like
9

This one’s also interesting: Let’s say I want t to be true if a<=b and b==3, but I make a typo.

julia> versioninfo()
Julia Version 0.6.0-pre.alpha.263
Commit 55f44179b9 (2017-03-27 00:01 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin13.4.0)
  CPU: Intel(R) Core(TM) i5-4690K CPU @ 3.50GHz
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Haswell)
  LAPACK: libopenblas64_
  LIBM: libopenlibm
  LLVM: libLLVM-3.9.1 (ORCJIT, haswell)

julia> t = (a <= b = 3)
3

julia> 1<=2

signal (11): Segmentation fault: 11
while loading no file, in expression starting on line 0
unknown function (ip: 0x31b7f2e83)
Allocations: 2846087 (Pool: 2844545; Big: 1542); GC: 3
Segmentation fault: 11
10

That shouldn’t crash and looks like a more fundamental problem with assigning function definitions. Reported as https://github.com/JuliaLang/julia/issues/21172.