Julia type graph generation v2

#------------------------------------------------------------------------------
#                          Julia Type Graphs
#------------------------------------------------------------------------------

## A short julia program that generates the graph of all subtypes
## of a given type, in png format by default. Uses graphviz.

## Examples: makegraph(Number, "result.dot")
##           => returns the name of the png file containing the Number subgraph.
##           makegraph(Any, "alltypes.dot", "svg")
##           => returns the name of the svg file containing all the types.

## Results:
## https://imgur.com/a/CkoOA#0
## https://imgur.com/U5vTDNp

## Applies recursively a function fn to typ and its subtypes.
## The collection of all encountered types are returned.
## Examples: length(typeit((x,y)-> nothing, Any))
##           typeit((x,y)-> "$x -> $y", Any)
##           length(typeit((x,y)-> println("$x -> $y"), Number))
function typeit(fn, typ)
   let seen  = Dict{Type,Bool}()
       apply = (fn, tp) -> if ~get(seen,tp,false)
                             seen[tp] = true
                             for child in subtypes(tp)
                               fn(tp,child)
                               apply(fn, child)
                             end
                           end
     apply(fn, typ)
     collect(keys(seen))
   end
end

## Graphviz doesn't like non alphanumeric characters, so I replace them with _.
## It does not like the type Graph either. So I make a label for this type.
function edge (typ1, typ2)
  let convert = typ -> map(x-> if isalnum(x) x else '_' end, string(typ))
      c1 = convert(typ1)
      c2 = convert(typ2)
   if c2 == "Graph" ; c2 = "G;\nG [label=\"Graph\"]" ; end
   "$(c1) -> $(c2);\n"
  end
end

graphviz_formats = ["png","gif","svg","jpg","svgz","pcl", "mif",
                    "pcl", "dia", "ps","fig", "imap","cmapx"]

## Produces two files named file and file.png. file is in graphviz dot format.
## The png file contains the graph of the subtypes of typ.
## The third argument is the output file format, png by default.
function makegraph(typ, file, format=png)
  if !contains(graphviz_formats, format)
    println("Warning: the output format may not be supported by graphviz.",
            "Here are the default formats: $(join(graphviz_formats, " "))")
  end
  fh = open(file, "w")
  write(fh, "digraph{\n")                # oriented graph
  write(fh, "rankdir=LR\n")              # layout of the graph from left to right
  typeit((x,y)-> write(fh, edge(x,y)), typ)
  write(fh,"}")
  close(fh)
  run(`dot -T$(format) -O $file`)        # graphviz needed here.
  print("$file.$format")
end