Define default TreeCharSet with global var instead of no-arg constructor

Author: jlumpe

docs/src/api.md

@@ -25,4 +25,6 @@ AbstractTrees.printnode
 AbstractTrees.siblinglinks
 treemap
 treemap!
+AbstractTrees.DEFAULT_CHARSET
+AbstractTrees.ASCII_CHARSET
 ```

src/printing.jl

@@ -96,9 +96,10 @@ struct TreeCharSet
     trunc
 end
 
-# Default charset
-TreeCharSet() = TreeCharSet('├','└','│','─','⋮')
-TreeCharSet(mid, term, skip, dash) = TreeCharSet(mid, term, skip, dash, '⋮')
+"""Default `charset` argument used by [`print_tree`](@ref)."""
+const DEFAULT_CHARSET = TreeCharSet('├', '└', '│', '─', '⋮')
+"""Charset using only ASCII characters."""
+const ASCII_CHARSET = TreeCharSet("+", "\\", "|", "--", "...")
 
 
 """
@@ -116,7 +117,7 @@ function print_prefix(io::IO, depth::Int, charset::TreeCharSet, active_levels)
 end
 
 function _print_tree(printnode::Function, io::IO, tree; maxdepth = 5, indicate_truncation = true,
-                     depth = 0, active_levels = Int[], charset = TreeCharSet(), withinds = false,
+                     depth = 0, active_levels = Int[], charset = DEFAULT_CHARSET, withinds = false,
                      inds = [], from = nothing, to = nothing, roottree = tree)
     nodebuf = IOBuffer()
     isa(io, IOContext) && (nodebuf = IOContext(nodebuf, io))

test/printing.jl

@@ -34,7 +34,7 @@ AbstractTrees.children(::SingleChildInfiniteDepth) = (SingleChildInfiniteDepth()
     #       └─ 3
     #
 
-    truncation_char = AbstractTrees.TreeCharSet().trunc
+    truncation_char = AbstractTrees.DEFAULT_CHARSET.trunc
 
     for maxdepth in [3,5,8]
         ptxt = repr_tree(Num(0), maxdepth=maxdepth)