add promises

Add promises to support awaiting async operations.

autoload/go/promise.vim

@@ -0,0 +1,50 @@
+" don't spam the user when Vim is started in Vi compatibility mode
+let s:cpo_save = &cpo
+set cpo&vim
+
+scriptencoding utf-8
+
+" New returns a promise. A promise's primary purpose is to make async jobs
+" synchronous by awaiting fn.
+"
+" A promise is a dictionary with two keys:
+"   'wrapper':
+"     A function that wraps fn. It can be used in place of fn.
+"   'await':
+"     A function that waits for wrapper to be called and returns the value
+"     returned by fn. Returns default if timeout expires.
+function! go#promise#New(fn, timeout, default) abort
+  let l:state = {}
+
+  " explicitly bind to state so that within l:promise's methods, self will
+  " always refer to state. See :help Partial for more information.
+  return {
+        \ 'wrapper': function('s:wrapper', [a:fn], l:state),
+        \ 'await': function('s:await', [a:timeout, a:default], l:state),
+  \ }
+endfunction
+
+function! s:wrapper(fn, ...) dict
+  let self.retval = call(a:fn, a:000)
+  return self.retval
+endfunction
+
+function! s:await(timeout, default) dict
+  let l:timer = timer_start(a:timeout, function('s:setretval', [a:default], self))
+  while !has_key(self, 'retval')
+    sleep 50m
+  endwhile
+  call timer_stop(l:timer)
+
+  return self.retval
+endfunction
+
+function! s:setretval(val, timer) dict
+  let self.retval = a:val
+endfunction
+
+" restore Vi compatibility settings
+let &cpo = s:cpo_save
+unlet s:cpo_save
+
+" vim: sw=2 ts=2 et

autoload/go/promise_test.vim

@@ -0,0 +1,41 @@
+" don't spam the user when Vim is started in Vi compatibility mode
+let s:cpo_save = &cpo
+set cpo&vim
+
+func! Test_PromiseNew() abort
+	let l:sut = go#promise#New(function('s:work', []), 100, -1)
+	call assert_true(has_key(l:sut, 'wrapper'))
+	call assert_true(has_key(l:sut, 'await'))
+endfunc
+
+func! Test_PromiseAwait() abort
+	let l:expected = 1
+	let l:default = -1
+	let l:sut = go#promise#New(function('s:work', [l:expected]), 100, l:default)
+
+	call timer_start(10, l:sut.wrapper)
+
+	let l:actual = call(l:sut.await, [])
+	call assert_equal(l:expected, l:actual)
+endfunc
+
+func! Test_PromiseAwait_Timeout() abort
+	let l:desired = 1
+	let l:expected = -1
+	let l:sut = go#promise#New(function('s:work', [l:desired]), 10, l:expected)
+
+	call timer_start(100, l:sut.wrapper)
+
+	let l:actual = call(l:sut.await, [])
+	call assert_equal(l:expected, l:actual)
+endfunc
+
+func! s:work(val, timer)
+	return a:val
+endfunc
+
+" restore Vi compatibility settings
+let &cpo = s:cpo_save
+unlet s:cpo_save
+
+" vim: sw=2 ts=2 et