nonblocking fixes

1. Throw an error if more than one default case
2. Always check default case last regardless of written position
3. Allow resulting expression to be anything (previously it had to
evaluate to a valid rvalue)

src/Select.jl

@@ -149,43 +149,47 @@ _take!(x) = wait(x)
 # since "wait" is level-triggered.
 _isready(c::AbstractChannel) = isready(c)
 _isready(t::Task) = istaskdone(t)
+
+# helper function to place the default case in the proper position
+function set_default_first!(clauses)
+    default_pos = find(clauses) do x
+        clause, body = x
+        clause.kind == SelectDefault
+    end
+    l = length(default_pos)
+    l == 0 && return # bail out if there is no default case
+    l  > 1 && throw(ErrorException("Select takes at most one default case. Found: $l"))
+    # swap elements to sure make SelectDefault comes first
+    clauses[1], clauses[default_pos[1]] = clauses[default_pos[1]], clauses[1]
+    clauses
+end
+
 function _select_nonblock_macro(clauses)
+    set_default_first!(clauses)
     branches = Expr(:block)
     for (clause, body) in clauses
-        local branch
+        branch =
         if clause.kind == SelectPut
-            branch = quote
-                if isready_put($(clause.channel|>get|>esc))
-                    put!($(clause.channel|>get|>esc), $(clause.value|>get|>esc))
-                    ret = $(esc(body))
-                    break
-                end
-            end
+            :(if isready_put($(clause.channel|>get|>esc))
+                put!($(clause.channel|>get|>esc), $(clause.value|>get|>esc))
+                $(esc(body))
+            end)
         elseif clause.kind == SelectTake
-            branch = quote
-                if _isready($(clause.channel|>get|>esc))
-                    $(clause.value|>get|>esc) =
-                    _take!($(clause.channel|>get|>esc))
-                    ret = $(esc(body))
-                    break
-                end
-            end
+            :(if _isready($(clause.channel|>get|>esc))
+                $(clause.value|>get|>esc) = _take!($(clause.channel|>get|>esc))
+                $(esc(body))
+            end)
         elseif clause.kind == SelectDefault
-            branch = quote
-                ret = $(esc(body))
-                break
-            end
+            :($(esc(body)))
         end
-        push!(branches.args, branch)
-    end
-    quote
-        local ret
-        while true
-            $branches
-        end
-        ret
+
+        # the next two lines build an if / elseif chain from the bottom up
+        push!(branch.args, branches)
+        branches = branch
     end
+    :($branches)
 end
+
 # The strategy for blocking select statements is to create a set of "rival"
 # tasks, one per condition. When a rival "wins" by having its conditional be
 # the first available, it sends a special interrupt to its rivals to kill them.
@@ -251,7 +255,7 @@ function _select_block_macro(clauses)
         push!(branches.args, branch)
 
         body_branch = :(if branch_id == $i; $bind_variable; $(esc(body)); end)
-        # the next two lines build an if / elseif chain
+        # the next two lines build an if / elseif chain from the bottom up
         push!(body_branch.args, body_branches)
         body_branches = body_branch
     end