ghci command to run top-level TH. Thanks /u/affinehyperplane for the trick! https://www.reddit.com/r/haskell/comments/18vkivp/comment/kh9byjd/?utm_source=reddit&utm_medium=web2x&context=3

th-ghci

:{
let doTH code = do
      u <- Data.Unique.newUnique
      let decls = "ghciTH_" ++ show (Data.Unique.hashUnique u)
      pure $ unlines
        [ unwords ["let", decls, "=", code]
        , "() = ();" ++ decls
        ]
:}

:def th doTH

Example usage (to inspect an apecs world):

λ :script ./th-ghci
λ :seti -XTemplateHaskell -XFlexibleInstances -XMultiParamTypeClasses
λ import Apecs
λ :th makeWorldAndComponents "World" []
λ :i World
type World :: *
data World
  = World {rec_6989586621679020007 :: !(Storage EntityCounter)}
        -- Defined at <interactive>:5:9
instance [safe] Monad m => Has World m EntityCounter
  -- Defined at <interactive>:5:9

Note that if there's an error in the code you pass to :th, it'll print the error and then a spurious error regarding ghciTH_XXX not being in scope:

λ :th makeWorldAndComponents "World" []

<interactive>:3:16: error:
    Variable not in scope:
      makeWorldAndComponents :: String -> [a0] -> t

<interactive>:3:9: error:
    Variable not in scope:
      ghciTH_1 :: Language.Haskell.TH.Lib.Internal.DecsQ

(I forgot to import Apecs in the above example)