MTL-style circuit simulator

After the first attempt at making a very side-effectful circuit simulator, and then an ambitious attempt at making it pure, I was reminded of a middle ground, using a pattern common in production Haskell code. It is called MTL-style effects, and it is a specific type of final tagless encoding.

The idea is that we first find a set of operations that our circuit simulator needs to support. Drawing this boundary well requires some experience and taste. Then we encode that limited set of operations into a type class. Here’s one way to do it.

In[1]:

class CircuitOperations m where
 make_wire :: m Wire
 get_signal :: Wire -> m Signal
 set_signal :: Wire -> Signal -> m ()
 add_action :: Wire -> m () -> m ()
 after_delay :: Int -> m () -> m ()
 get_time :: m Int
 probe_output :: String -> m ()
 propagate :: m ()

Having read the previous article, these operations should sound familiar. I’m sure it’s possible to create a better set of operations, but this will suffice to illustrate the point.