The code in the previous post can do with a bit of cleaning up. I start with introducing a type class for an API that can be run

class RunnableF f where
  runOp :: f a -> IO a

and a function that actually runs iteFile

runF :: (RunnableF o) => Free o a -> IO a
runF = foldFree runOp

A coproduct (Sum) of two runnable APIs can itself be runnable

instance (RunnableF f, RunnableF g) => RunnableF (Sum f g) where
  runOp (InL op) = runOp op
  runOp (InR op) = runOp op

After this all I have to do is to move the guts of runSimpleF and runLogging into implementations for SimpleFil and LogF respectively

instance RunnableF SimpleFileF where
  runOp (LoadFile fp f') = liftM f' $ readFile fp
  runOp (SaveFile fp d r) = writeFile fp d >> return r

instance RunnableF LogF where
  runOp (Log s a)= putStrLn s >> return a

The rest is left as is. Running now looks like this

> :! cat test.txt
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec a diam lectus.
Sed sit amet ipsum mauris. Maecenas congue ligula ac quam viverra nec
consectetur ante hendrerit.
> runF $ withSimpleFile (map toUpper) "test.txt"
> :! cat test.txt_new
LOREM IPSUM DOLOR SIT AMET, CONSECTETUR ADIPISCING ELIT. DONEC A DIAM LECTUS.
SED SIT AMET IPSUM MAURIS. MAECENAS CONGUE LIGULA AC QUAM VIVERRA NEC
CONSECTETUR ANTE HENDRERIT.
> runF $ foldFree loggingSimpleFileI $ withSimpleFile (map toLower) "test.txt"
** load file test.txt
** save file test.txt_new
> :! cat test.txt_new
lorem ipsum dolor sit amet, consectetur adipiscing elit. donec a diam lectus.
sed sit amet ipsum mauris. maecenas congue ligula ac quam viverra nec
consectetur ante hendrerit.

What else?

With these changes it becomes slightly esier to create new basic API types. but there are still a few things I think could be useful:

• More generic decorators. logSimpleFileI is tied to decorating only SimpleFileF. I’m fairly sure this could be dealt with by a type class LoggableF and have APIs implement it. I also think there’s a rather natural implementation of LoggableF for Sum f g.
• Combining decorators (and other interpreters). I’m guessing this is what John refers to as “generalizing interpreters.”
• Combining APIs. I’d prefer making small APIs of related operations, but then that also means I need a nice way of combining APIs. I had a go at using FreeT but simply gave up on bending the types to my will. In any case I’m not completely sure that a stack of various FreeT is a good direction to go.