From 3afc5c19d6ce253a1519c0be1126c87627fd5ce5 Mon Sep 17 00:00:00 2001
From: Brett Cornell <brett.durcan@gmail.com>
Date: Sat, 31 Oct 2015 16:46:17 -0700
Subject: [PATCH] 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 | 62 +++++++++++++++++++++++++++------------------------
 1 file changed, 33 insertions(+), 29 deletions(-)

diff --git a/src/Select.jl b/src/Select.jl
index 83426c8..d85a504 100644
--- a/src/Select.jl
+++ b/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