Made first argument in lume.reduce() optional

README.md

@@ -105,12 +105,14 @@ supplied it is called on each value, true is returned if any of the calls to
 lume.any({1, 2, 1}, function(x) return x == 1 end) -- Returns true
 ```
 
-### lume.reduce(t, fn, first)
+### lume.reduce(t, fn [, first])
 Applies `fn` on two arguments cumulative to the items of the array `t`, from
-left to right, so as to reduce the array to a single value. The accumulator is
-intialised to the `first` value.
+left to right, so as to reduce the array to a single value. If a `first` value
+is specified the accumulator is initialised to this, otherwise the first value
+in the array is used. If the array is empty and no `first` value is specified
+an error is raised,
 ```lua
-lume.reduce({1, 2, 3}, function(a, b) return a + b end, 0) -- Returns 6
+lume.reduce({1, 2, 3}, function(a, b) return a + b end) -- Returns 6
 ```
 
 ### lume.set(t [, retainkeys])

lume.lua

@@ -119,8 +119,10 @@ end
 
 
 function lume.reduce(t, fn, first)
-  for i = 1, #t do first = fn(first, t[i]) end
-  return first
+  local acc = first or t[1]
+  assert(acc, "reduce of an empty array with no first value")
+  for i = first and 1 or 2, #t do acc = fn(acc, t[i]) end
+  return acc
 end