interface: introduce maxsteps

In case of CartPole v0 and v1,
there is a limitation of steps in single episode.

close #16

src/Reinforce.jl

@@ -101,6 +101,13 @@ The `ismdp` query returns true when the environment is MDP, and false otherwise.
 """
 ismdp(env::AbstractEnvironment) = false
 
+"""
+    maxsteps(env)::Int
+
+Return the max steps in single episode.
+Default is `0` (unlimit).
+"""
+maxsteps(env::AbstractEnvironment) = 0
 
 # ----------------------------------------------------------------
 # Implement this interface for a new policy

src/envs/cartpole.jl

@@ -20,11 +20,17 @@ const x_threshold = 2.4
 mutable struct CartPole <: AbstractEnvironment
 	state::Vector{Float64}
 	reward::Float64
+  maxsteps::Int  # max step in each episode
 end
-CartPole() = CartPole(0.1rand(4)-0.05, 0.0)
+CartPole(; maxsteps = 0) = CartPole(0.1rand(4)-0.05, 0.0, maxsteps)
 
-reset!(env::CartPole)  = (env.state = 0.1rand(4)-0.05; env.reward = 0.0; return)
+# see https://github.com/FluxML/model-zoo/pull/23#issuecomment-366030179
+CartPoleV0() = CartPole(maxsteps = 200)
+CartPoleV1() = CartPole(maxsteps = 500)
+
+reset!(env::CartPole) = (env.state = 0.1rand(4)-0.05; env.reward = 0.0; return)
 actions(env::CartPole, s) = DiscreteSet(1:2)
+maxsteps(env::CartPole) = env.maxsteps
 
 function step!(env::CartPole, s, a)
   s = state(env)

src/episodes/iterators.jl

@@ -26,7 +26,9 @@ function Base.start(ep::Episode)
 end
 
 function Base.done(ep::Episode, i)
-    finished(ep.env, state(ep.env))
+  n = maxsteps(ep.env)
+  (n != 0 && ep.niter >= n) && return true
+  finished(ep.env, state(ep.env))
 end
 
 # take one step in the enviroment after querying the policy for an action