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If you are willing to slightly modify the original interpreter (by changing how the terminal case is handled) <pre><code>{-# LANGUAGE DeriveFunctor #-} import Control.Monad.Free import Control.Monad.Morph import Pipes data FooF a = Foo String a | Bar Int a deriving (Functor) printFoo :: Free FooF a -> IO a printFoo (Free (Foo s n)) = print s >> printFoo n printFoo (Free (Bar i n)) = print i >> printFoo n printFoo (Pure a) = return a </code></pre> ...then there's a way to add extra actions without modifying the functor or having to repurpose its constructors, while still being able to reuse the interpreter. The solution uses the <code>pipes</code> and <code>mmorph</code> packages. First you have to define a sort of "pre-interpeter" that lifts the free monad into a <code>Producer</code> from <code>pipes</code>. The <code>yield ()</code> statements in the producer signify the points at which an extra action is inserted. <pre><code>pre :: Free FooF a -> Producer () (Free FooF) a pre (Free (Foo s n)) = lift (Free . Foo s $ return ()) >> yield () >> pre n pre (Free (Bar i n)) = lift (Free . Bar i $ return ()) >> yield () >> pre n pre (Pure a) = lift . Pure $ a </code></pre> (In a more complex example the <code>yield</code> statements could carry extra information, like log messages.) Then you write a function that applies the <code>printFoo</code> interpreter underneath the <code>Producer</code>, using <code>hoist</code> from <code>mmorph</code>: <pre><code>printFooUnder :: Producer () (Free FooF) a -> Producer () IO a printFooUnder = hoist printFoo </code></pre> So, we have a function that "interprets" the free monad into <code>IO</code>, but at some points emits <code>()</code> values that we must decide how to handle. Now we can define an extended interpreter that reuses the old interpreter: <pre><code>printFooWithReuse :: Show a => Free FooF a -> IO () printFooWithReuse foo = do finalv <- runEffect $ for (printFooUnder . pre $ foo) (\_ -> lift (print "extra info")) print finalv </code></pre> After testing it, it seems to work: <pre><code>printFooWithReuse $ Free (Foo "nah" (Pure 4)) -- > "nah" -- > "extra info" -- > 4 </code></pre> If you happen to want to insert the extra actions manually, then your can eschew writing the "pre-interpreter" and work directly in the <code>Producer () (Free FooF)</code> monad. (This solution could also be achieved by layering a free monad transformer instead of a <code>Producer</code>. But I think using a <code>Producer</code> is a bit easier.)
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