Update globals to encourage type-annotation (#2)

Author: timholy

test/globals.jl

@@ -8,7 +8,8 @@ using Test
     # in the initial view (otherwise, you can hit 'w' to toggle "warn" mode).
     # In this case, make sure you know the inferred types for `idx` and `charset`.
     # Once diagnosed, fix the code in src/globals.jl. To fix it, read
-    # https://docs.julialang.org/en/v1/manual/performance-tips/#Avoid-global-variables
+    # https://docs.julialang.org/en/v1/manual/performance-tips/#Avoid-untyped-global-variables
+    # Here you can assume that the value of `charset` never needs to change.
     @test @inferred(getchar(5)) == 'e'   # see `?@inferred`
     # Check `@descend` again at the end to see that you've fixed it!
     # (Chances are you'll have to restart Julia even if you're using Revise; there are some changes
@@ -18,11 +19,8 @@ using Test
     @test get_homedir() == "wrong"
     set_homedir(ENV["HOME"])
     @test get_homedir() == ENV["HOME"]
-    # This one will fail. But you can't make the value constant and still update it!
-    # Find a way to fix it. Hint: `?Ref` or use an array (Refs are like single-element arrays)
-    # Explanation: the *container* can be constant, but the *contents* can be modified.
-    # (Think of the container like a box: you can take something out of the box and put something different
-    #  inside it, but the box itself is unchanged.)
-    # If the container type specifies the type of the contents, Julia's type-inference will be happy.
+    # The next one will fail.
+    # Be sure you follow the link on global-value type annotations in
+    #   https://docs.julialang.org/en/v1/manual/performance-tips/#Avoid-untyped-global-variables
     @test @inferred(get_homedir()) == ENV["HOME"]   # again, this will likely require restarting Julia to test your fix
 end