hs.fnutils.reduce takes an optional initial value

This is in line with the reduce function in other languages like JavaScript, Python, and Ruby.

extensions/fnutils/fnutils.lua

@@ -225,27 +225,34 @@ function fnutils.mapCat(t, fn)
   return nt
 end
 
---- hs.fnutils.reduce(table, fn) -> table
+--- hs.fnutils.reduce(table, fn[, initial]) -> table
 --- Function
 --- Reduce a table to a single element, using a function
 ---
 --- Parameters:
 ---  * table - A table containing some sort of data
 ---  * fn - A function that takes two parameters, which will be elements of the supplied table. It should choose one of these elements and return it
+---  * initial - If given, the first call to fn will be with this value and the first element of the table
 ---
 --- Returns:
 ---  * The element of the supplied table that was chosen by the iterative reducer function
 ---
 --- Notes:
 ---  * table cannot be a sparse table, see [http://www.luafaq.org/gotchas.html#T6.4](http://www.luafaq.org/gotchas.html#T6.4)
 ---  * The first iteration of the reducer will call fn with the first and second elements of the table. The second iteration will call fn with the result of the first iteration, and the third element. This repeats until there is only one element left
-function fnutils.reduce(t, fn)
+function fnutils.reduce(t, fn, ...)
+  local rest = {...}
   local len = #t
-  if len == 0 then return nil end
-  if len == 1 then return t[1] end
-
-  local result = t[1]
-  for i = 2, #t do
+  local start, result
+  if #rest == 0 then
+    if len == 0 then return nil end
+    result = t[1]
+    start = 2
+  else
+    result = rest[1]
+    start = 1
+  end
+  for i = start, len do
     result = fn(result, t[i])
   end
   return result

extensions/fnutils/test_fnutils.lua

@@ -0,0 +1,10 @@
+function test_reduce()
+  local reduce = hs.fnutils.reduce
+
+  assert(reduce({}, function(x, y) return x + y end) == nil)
+  assert(reduce({}, function(x, y) return x + y end, 10) == 10)
+  assert(reduce({1}, function(x, y) return x + y end) == 1)
+  assert(reduce({1}, function(x, y) return x + y end, 10) == 11)
+  assert(reduce({1, 2}, function(x, y) return x + y end) == 3)
+  assert(reduce({1, 2}, function(x, y) return x + y end, 10) == 13)
+end