Define meta and replace.

[JeffreyBenjaminBrown]

meta lets you apply a pattern whose elements are pattern transformations like fast 2. replace makes it easier to use meta, and probably has other uses; it lets you turn a pattern of easy-to-write symbols (integers are what I usually use) into a pattern of anything else, by providing a lookup table.

+meta :: forall a b.

•    Pattern (Pattern a -> Pattern b) -> Pattern a -> Pattern b
  

+replace :: forall a b. Ord a =>

•       [(a,b)] -> Pattern a -> Pattern b
  

Here's a minimal example using both.

This plays the pattern "ho*8"
at normal speed for the first half,
and at double speed for the second half.

let slang = [ (0, id)
, (1, fast 2) ]
in p 1 $ meta (replace slang "0 1") "ho*8"

[JeffreyBenjaminBrown]

On the forum, user @mvdirty managed to make one-liners out of both functions. His definition of meta is beautiful, and more general than mine:

meta :: Monad m => m (a -> m b) -> a -> m b
meta pf p = pf >>= ($ p)

It's simple enough that I no longer think meta deserves to exist.

His definition of replace is also beautiful, but complex enough that maybe it deserves to be defined in the codebase:

replace :: Show a => [(a, b)] -> Pattern String -> Pattern b
replace x = inhabit $ bimap show pure <$> x

The code I submitted is basically irrelevant at this point, but I'm leaving this pull request open just for discussion's sake.