Unexpected 'map' behaviour


#1

I was playing around with map and encountered some unexpected behaviour when changing the definition of the argument function.

Define a function

julia> f(x) = x + 2
f (generic function with 1 method)

Apply map to the function and a collection.

julia> map(f,1:3)
3-element Array{Int64,1}:
 3
 4
 5

Redefine f

f(x) = x + 3
WARNING: Method definition f(Any) in module Main at REPL[1]:1 overwritten at REPL[3]:1.
f (generic function with 1 method)

Apply map to redefined f and the same collection.

julia> map(f,1:3)
3-element Array{Int64,1}:
 3
 4
 5

Hmmm. Looks like f never changed. Collect the collection.

map(f,collect(1:3))
3-element Array{Int64,1}:
 4
 5
 6

Oh, now it changed. Change f one more time.

f(x) = x + 4
WARNING: Method definition f(Any) in module Main at REPL[3]:1 overwritten at REPL[13]:1.
f (generic function with 1 method)

Apply map to f and the collected collection.

map(f,collect(1:3))
3-element Array{Int64,1}:
 4
 5
 6

It’s still using the previous definition of f. Well then.

I expected map to use the current definition of f, so if I change f and call map again same arguments as before, map would use the new definition of f, much like changing the second argument to map changes the collection that f gets applied to. But instead, it looks like map is using the definition of f from when map was first called.

This is not the kind of behaviour I would infer from the documentation which states simply that map(f,c...) “transform[s] collection c by applying f to each element.” I spent a while reading other documentation on functions to find the answer, but didn’t see anything describing or explaining this kind of behaviour.

I can get around this by writing map(x->f(x),r), but that seems rather roundabout.

Is this behaviour on purpose? Is there a better way to work around it?


#2

It looks like you’re still on Julia 0.5 or older. Update to Julia 0.6 to fix it.


#3

In case you’re curious, you’ve run into the dreaded issue 265: https://github.com/julialang/julia/issues/265 which was finally resolved in Julia 0.6, as mbauman mentioned.


#4

Ah, there we go! Thanks!