molly: add a log iterator

The patch add a generator which, when asked for an operation, logs
a message and yields `nil`.

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- A `log()` iterator.
 - A `mix()` iterator.
 - A CAS-register generator.
 

molly/gen.lua

@@ -38,6 +38,7 @@
 
 local fun = require('fun')
 local clock = require('molly.clock')
+local log = require('molly.log')
 local tbl = require('molly.compat.tbl')
 
 local fun_mt = debug.getmetatable(fun.range(10))
@@ -456,15 +457,29 @@ methods.flip_flop = flip_flop
 --- Special generators
 -- @section
 
---- (TODO) A generator which, when asked for an operation, logs a message and yields
---  nil. Occurs only once; use `repeat` to repeat.
+--- A generator which, when asked for an operation, logs
+-- a message and yields `nil`. Occurs only once; use `repeat` to
+-- repeat.
 -- @return an iterator
 --
+-- @usage
+-- > molly.gen.iter({1, 2, 3}):log():totable()
+-- 1
+-- ---
+-- - - 2
+--   - 3
+--   ...
+-- --
 -- @function log
-local log = function()
-    -- TODO
+local log_it = function(it)
+    assert(tostring(it) == '<generator>')
+    local gen, param, state = unwrap(it)
+    local state1, v = gen(param, state)
+    log.info(v)
+    return fun.wrap(gen, param, state1)
 end
-exports.log = log
+exports.log = log_it
+methods.log = log_it
 
 --- (TODO) Operations from that generator are scheduled at uniformly random intervals
 -- between `0` to `2 * (dt seconds)`.

test/tests.lua

@@ -27,7 +27,7 @@ local utils = molly.utils
 local seed = os.time()
 math.randomseed(seed)
 
-test:plan(11)
+test:plan(12)
 
 test:test('clock', function(test)
     test:plan(5)
@@ -199,6 +199,13 @@ test:test('gen.mix', function(test)
     test:is(#tbl, 11)
 end)
 
+test:test('gen.log', function(test)
+    test:plan(1)
+
+    local tbl = molly.gen.iter({1, 2, 3}):log():totable()
+    test:is(#tbl, 2)
+end)
+
 test:test('runner', function(test)
     test:plan(4)