patch 7.4.2044

Problem:    filter() and map() either require a string or defining a function.
Solution:   Support lambda, a short way to define a function that evaluates an
            expression. (Yasuhiro Matsumoto, Ken Takata)

src/testdir/test_lambda.vim

@@ -0,0 +1,48 @@
+function! Test_lambda_with_filter()
+  let s:x = 2
+  call assert_equal([2, 3], filter([1, 2, 3], {i, v -> v >= s:x}))
+endfunction
+
+function! Test_lambda_with_map()
+  let s:x = 1
+  call assert_equal([2, 3, 4], map([1, 2, 3], {i, v -> v + s:x}))
+endfunction
+
+function! Test_lambda_with_sort()
+  call assert_equal([1, 2, 3, 4, 7], sort([3,7,2,1,4], {a, b -> a - b}))
+endfunction
+
+function! Test_lambda_with_timer()
+  if !has('timers')
+    return
+  endif
+
+  let s:n = 0
+  let s:timer_id = 0
+  function! s:Foo()
+    "let n = 0
+    let s:timer_id = timer_start(50, {-> execute("let s:n += 1 | echo s:n")}, {"repeat": -1})
+  endfunction
+
+  call s:Foo()
+  sleep 200ms
+  " do not collect lambda
+  call test_garbagecollect_now()
+  let m = s:n
+  sleep 200ms
+  call timer_stop(s:timer_id)
+  call assert_true(m > 1)
+  call assert_true(s:n > m + 1)
+  call assert_true(s:n < 9)
+endfunction
+
+function! Test_lambda_with_partial()
+  let l:Cb = function({... -> ['zero', a:1, a:2, a:3]}, ['one', 'two'])
+  call assert_equal(['zero', 'one', 'two', 'three'], l:Cb('three'))
+endfunction
+
+function Test_lambda_fails()
+  call assert_equal(3, {a, b -> a + b}(1, 2))
+  call assert_fails('echo {a, a -> a + a}(1, 2)', 'E15:')
+  call assert_fails('echo {a, b -> a + b)}(1, 2)', 'E15:')
+endfunc