Add Promise.retry

Closes #16

lib/README.md

@@ -349,6 +349,26 @@ docs:
       returns: Promise<array<U>>
       static: true
 
+    - name: retry
+      desc: |
+        Repeatedly calls a Promise-returning function up to `times` number of times, until the returned Promise resolves.
+
+        If the amount of retries is exceeded, the function will return the latest rejected Promise.
+      params:
+        - name: callback
+          type:
+            kind: function
+            params: "...: P"
+            returns: Promise<T>
+        - name: times
+          type: number
+        - name: "..."
+          type: "P"
+          optional: true
+
+      returns: Promise<T>
+      static: true
+
     - name: is
       desc: Checks whether the given object is a Promise via duck typing. This only checks if the object is a table and has an `andThen` method.
       static: true

lib/init.lua

@@ -1323,4 +1323,22 @@ function Promise.prototype:now(rejectionValue)
 	end
 end
 
+--[[
+	Retries a Promise-returning callback N times until it succeeds.
+]]
+function Promise.retry(callback, times, ...)
+	assert(type(callback) == "function", "Parameter #1 to Promise.retry must be a function")
+	assert(type(times) == "number", "Parameter #2 to Promise.retry must be a number")
+
+  local args, length = {...}, select("#", ...)
+
+  return Promise.resolve(callback(...)):catch(function(...)
+    if times > 0 then
+      return Promise.retry(callback, times - 1, unpack(args, 1, length))
+    else
+      return Promise.reject(...)
+    end
+  end)
+end
+
 return Promise

lib/init.spec.lua

@@ -1410,4 +1410,35 @@ return function()
 			expect(innerPromise:getStatus()).to.equal(Promise.Status.Cancelled)
 		end)
 	end)
+
+	describe("Promise.retry", function()
+		it("should retry N times", function()
+			local counter = 0
+
+			local promise = Promise.retry(function(parameter)
+				expect(parameter).to.equal("foo")
+
+				counter = counter + 1
+
+				if counter == 5 then
+					return Promise.resolve("ok")
+				end
+
+				return Promise.reject("fail")
+			end, 5, "foo")
+
+			expect(promise:getStatus()).to.equal(Promise.Status.Resolved)
+			expect(promise._values[1]).to.equal("ok")
+		end)
+
+		it("should reject if threshold is exceeded", function()
+			local promise = Promise.retry(function()
+				return Promise.reject("fail")
+			end, 5)
+
+			expect(promise:getStatus()).to.equal(Promise.Status.Rejected)
+			expect(promise._values[1]).to.equal("fail")
+		end)
+	end)
+
 end