Implement ale.pad in Lua

doc/ale.txt

@@ -4498,7 +4498,8 @@ ale#Has(feature)                                                    *ale#Has()*
   `ale#Has('ale-x.y.z')`, such as `ale#Has('ale-2.4.0')`.
 
 
-ale#Pad(string)                                                     *ale#Pad()*
+ale.pad(str)                                                        *ale.pad()*
+ale#Pad(str)                                                        *ale#Pad()*
 
   Given a string or any |empty()| value, return either the string prefixed
   with a single space, or an empty string. This function can be used to build

lua/ale/init.lua

@@ -64,6 +64,21 @@ ale.has = function(feature)
     return vim.fn["ale#Has"](feature) == 1
 end
 
+---Prefix a string with a single space if it is not empty.
+---nil will be treated the same as an empty string.
+---
+---This function is a convenience for chaining options for commands together
+---without adding redundant whitespace.
+---@param str string|nil A value to pad with whitespace.
+---@return string padded A value padded with whitespace.
+ale.pad = function(str)
+    if str == nil or str == "" then
+        return ""
+    end
+
+    return " " .. str
+end
+
 ---Get an ALE variable for a buffer (first) or globally (second)
 ---@param buffer number The buffer number to retreive the variable for.
 ---@param variable_name string The variable to retrieve.

test/lua/ale_pad_spec.lua

@@ -0,0 +1,13 @@
+local eq = assert.are.same
+local ale = require("ale")
+
+describe("ale.pad", function()
+    it("should pad non-empty strings", function()
+        eq(" foo", ale.pad("foo"))
+    end)
+
+    it("should return empty strings for nil or empty strings", function()
+        eq("", ale.pad(nil))
+        eq("", ale.pad(""))
+    end)
+end)