Envelope documentation

[JeffreyBenjaminBrown]

A million years ago on an extinct mailing list (Haskell Art), you gave me an example of how to use envelopes. Yesterday I discovered it works like a charm. I've tweaked it a little and added comments. It might make a good piece of documentation to add somewhere, maybe at the top of Vivid.Envelopes.

{-# LANGUAGE DataKinds, ExtendedDefaultRules #-}

import Vivid

main :: IO ()
main = do
   s <- synth foo ()
   wait 1
   putStrLn "Now we fade in!"
   set s (1 ::I "gate")
   wait 4 -- because the envelope below lasts 1+2+1 seconds
   free s

e :: EnvLiterally '["gate"]
e = env 0 -- The level to start at.
  [ -- These are (level, time in seconds to reach that level) pairs.
    (1, 1) 
  , (1, 2) -- Since the last level was also 1, this pair causes
  -- the envelope to stay at 1 for 2 seconds after reaching it.
  , (0, 1)]
  Curve_Lin

-- This has a default gate of 0, so the envelope doesn't trigger when
-- the voice is created; rather, the user must send a positive gate value
-- to trigger it. (If they then send another 0, they gain the opportunity
-- to retrigger it with another positive gate value.)
-- If instead the default was 1, then the user would not need to send
-- the initial gate value separately (but they could still send 0
-- to make it ready to retrigger).
foo :: SynthDef '["gate"]
foo = sd (0 ::I "gate") $ do
   e' <- envGen_wGate (V::V "gate") 1 e DoNothing
   s <- sinOsc (freq_ 440) ~* e'
   out 0 [s,s]