add a throttle macro

src/utils.jl

@@ -551,8 +551,9 @@ _eltype(::AbstractArray{T}) where T = T
 """
     throttle(f, timeout; leading=true, trailing=false)
 
-Return a function that when invoked, will only be triggered at most once
-during `timeout` seconds.
+Return a function that when called, will only call the given `f` at most
+once during `timeout` seconds. Any arguments passed to this new function
+are passed to `f`.
 
 Normally, the throttled function will run as much as it can, without ever
 going more than once per `wait` duration; but if you'd like to disable the
@@ -561,17 +562,27 @@ the trailing edge, pass `trailing=true`.
 
 # Examples
 ```jldoctest
-julia> a = Flux.throttle(() -> println("Flux"), 2);
+julia> noarg = Flux.throttle(() -> println("Flux"), 2);
 
-julia> for i = 1:4  # a called in alternate iterations
-           a()
+julia> for i in 1:4
+           noarg()   # println called in alternate iterations
            sleep(1)
        end
 Flux
 Flux
+
+julia> onearg = Flux.throttle(i -> println("step = ", i), 1);
+
+julia> for i in 1:10
+           onearg(i)
+           sleep(0.3)
+       end
+step = 1
+step = 5
+step = 9
 ```
 """
-function throttle(f, timeout; leading=true, trailing=false)
+function throttle(f, timeout::Real; leading=true, trailing=false)
   cooldown = true
   later = nothing
   result = nothing
@@ -603,6 +614,44 @@ function throttle(f, timeout; leading=true, trailing=false)
   end
 end
 
+"""
+    @throttle timeout expr
+
+Evaluates the given expression at most once every `timeout` seconds.
+
+Internally, it uses [`throttle`](@ref Flux.throttle). But instead of
+defining a function outside the loop, it lets you place the code inside
+the loop.
+
+# Example
+```jldoctest
+julia> for i in 1:20
+         j = 100i
+         sleep(0.2)
+         Flux.@throttle 0.9 if iseven(i)
+           println("i = ", i, ", and j = ", j)
+         else
+           println("i = ", i)
+         end
+       end
+i = 1
+i = 6, and j = 600
+i = 11
+i = 16, and j = 1600
+```
+"""
+macro throttle(timeout::Real, ex)
+  expr = macroexpand(__module__, ex)
+  vars = unique(_allsymbols(expr))
+  @gensym fast slow
+  Base.eval(__module__, :($fast($(vars...)) = $expr))
+  Base.eval(__module__, :(const $slow = $throttle($fast, $timeout)))
+  :($slow($(vars...))) |> esc
+end
+
+_allsymbols(s::Symbol) =[s]
+_allsymbols(other) = Symbol[]
+_allsymbols(ex::Expr) = vcat(_allsymbols.(ex.args)...)
 
 """
     modules(m)
@@ -675,7 +724,6 @@ julia> loss() = rand();
 
 julia> trigger = Flux.patience(() -> loss() < 1, 3);
 
-
 julia> for i in 1:10
          @info "Epoch \$i"
          trigger() && break