Amusingly, just like whoever wrote the Haskell example linked in the blog, the very first thing I did with free monads was also write a trivial stack-based lang

Amusingly, just like whoever wrote the Haskell example linked in the blog, the very first thing I did with free monads was also write a trivial stack-based language. All I supported was pushing numbers to the stack and addition. Not a very complicated language! Programs look something like: prog = do {1; 2; add; 3; 4; add; add} The cool thing is that I get "subroutines" (or, as Forth calls them, "words") for free: incr = do {1; add} prog = do {1; incr; incr; incr} The interesting thing is that incr and prog are actually ASTs: I can run them, with a function called runProgram, but I can just as easily pretty print them. This is a wonderful property of free monads that's very useful for real DSLs. And, in fact, I'm using them for a current project--they're great. With a bit of cleverness, it's probably possible to encode some of the stack semantics into the type system, making running a program with too few or too many elements left on the stack a type error. Also, with GADTs, I can probably add support for something like strings and have a mini string/int type system just for my little language, without much effort.