disallow methods of local functions in different blocks

[wbhart]

In Julia 0.5.0-dev+3171 the following code creates a bogus duplicate function definition warning.

function myfun(n::Int)
     if n < 3
        doit() = 3
     else
        doit() = 5
     end 
end

[JeffBezanson]

This isn't allowed any more; I'll have to come up with a better error message. Now you need to do

function myfun(n::Int)
     if n < 3
        doit = ()->3
     else
        doit = ()->5
     end 
end

[ChrisRackauckas]

Is there a reason why this is no longer allowed?

[JeffBezanson]

Yes; this restriction makes it possible to hoist construction of f's method table to the top level. A clearer example is

function myfun()
    f() = 0
    if rand(Bool)
        f(x::Int) = 1
    end
    f(...)
end

Here, f would sometimes have one method and sometimes have two methods. That would require rebuilding f's method table on each call to myfun, which is too slow to be useful. The work-around is to use separate functions, as in my comment above, or

function myfun()
    f1() = 0
    f2() = f1()
    f2(x::Int) = 1
    f = rand(Bool) ? f2 : f1
    f(...)
end

[ChrisRackauckas]

Yeah, I found those workarounds. Thanks for explaining it. Now that I know there's a reason it's easier to remember.

I think that this should say something like Warning: functions cannot be conditionally defined. Last method definition is chosen for each dispatch. and then have a link to somewhere in documentation which elaborates more on it (I think there should be a whole page which elaborates on errors and warnings, especially for more obscure ones like this which are best explained via examples).

However, to make it a little more user friendly, my example

function g(a)
  if a
    f() = 2
  else
    f() = 3
  end
  return f
end
f = g(true)
f() # Returns 3

shouldn't error with g(false), but should just have the same answer as g(true) and give the same warning.

[ViralBShah]

Perhaps this should be documented in NEWS.md as @josefsachsconning pointed out.

[josefsachsconning]

Documentation pretty crucial, since this is a breaking change.