In a recent blog by John Myles White
http://www.johnmyleswhite.com/notebook/2014/09/06/values-vs-bindings-the-map-is-not-the-territory/
it was suggested that Julia make strong distinction between values and
binding. For example
a = [1, 2, 3]
function foo!(a)
a[1] = 10
return
end
a
Would give an output
3-element Array{Int64,1}:
10
2
3
But subsequently running
function bar!(a)
a = [1, 2]
return
end
a
would give an output of
3-element Array{Int64,1}:
10
2
3
The reason why the variable "a" was not changed in the case of "boo!", but
was changed in the case of "foo!" has to do with the fact values are the
same their binding.
However to illustrate that point, the examples given in the blog are quite
erroneous. For example
[1,2,3]==[1,2,3]
would yield an out of "true", whereas the blog post indicated that is
should be "false". This the because the "==" operator unpacks the values in
the binding and does and element wise comparison.
A better was illustrate that problem is make use of julia's "object_id"
function. For example
a=[1,2,3]
@printf("Object ID in outerscope 0x%x\n", object_id(a))
function bar!(a)
@printf("Object ID in \!bar and before assignment 0x%x\n", object_id(a))
a = [1, 2]
@printf("Object ID in \!bar after assignment 0x%x\n", object_id(a))
return
end
bar!(a)
@printf("Object ID in outerscope after \!bar is 0x%x\n", object_id(a))
a
The output will be
Object ID in outerscope 0xa19caeb70115d4f0
Object ID in !bar and before assignment 0xa19caeb70115d4f0
Object ID in !bar after assignment 0x782193112dfbbbd5
Object ID in outerscope after !bar is 0xa19caeb70115d4f0
Out[1]:
3-element Array{Int64,1}:
1
2
3
This would mean the when calling a function in Julia the following is true:
- The binding of the argument to the function can never change, though
the values of that binding might.
- The binding can be overridden locally in the scope of the function,
but that is only valid in inside the function's scope
A modification to the above code that does not attempt to override the
original binding, shown below would not change the object id.
a=[1,2,3]
@printf("Object ID in outerscope 0x%x\n", object_id(a))
function bar!(a)
@printf("Object ID in \!bar and before assignment 0x%x\n", object_id(a))
empty!(a)
push!(a,[1, 2]...)
@printf("Object ID in \!bar after assignment 0x%x\n", object_id(a))
return
end
bar!(a)
@printf("Object ID in outerscope after \!bar is 0x%x\n", object_id(a))
a
Is this how julia handles values and bindings, or it there more to this
picture?
