julialang type graph generation

#################################################################
## A short julia program that generates the graph of all subtypes
## of a given type, in png format. Uses graphviz.
## Example: makegraph(Number, "result.dot")
## returns a png file named result.dot.png that contains the Number subgraph.
## https://imgur.com/a/CkoOA#0

## Applies recursively a function fn to typ and its subtypes.
## The collection of all encountered types are returned.
## Example: length(typeit((x,y)-> println("$x -> $y"), Number))
function typeit(fn, typ)
   let seen = Dict{Type,Bool}()        # returns true for already seen types
       doit = function rec (fn, tp)
                if ~get(seen,tp,false)
                  seen[tp] = true
                  for child in subtypes(tp)
                    fn(tp,child)
                    rec(fn, child)
                  end
                end
              end
     doit(fn, typ)
     collect(keys(seen))
   end
end

## graphviz doesn't like non alphanumeric characters.
function edge (typ1, typ2)
  if all(isalnum,string(typ1)) & all(isalnum,string(typ2))
  "$typ1 -> $typ2;\n"
  else ""
  end
end

## Produces two files named file and file.png. file is in graphviz dot format.
## The png file contains the graph of the subtypes of the IO type.
function makegraph(typ, file)
  fh = open(file, "w")
  write(fh, "digraph{\n")
  typeit((x,y)-> write(fh, edge(x,y)), typ)
  write(fh,"}")
  close(fh)
  run(`dot -Tpng -O $file`)        # graphviz needed here.
  print("$file.png")
end