The main challenge of writing a lazy interpreter is sharing structure: in particular, making sure that an individual closure is not evaluated more than once. Obvious but tedious solutions in Haskell include using IORefs and monadic state. The interpreter below uses a completely different tactic: exploiting unsafeInterleaveIO. All function arguments are evaluated “right away”, but in the context of an unsafeInterleaveIO (so, in fact, they are actually not evaluated right away). With this hack, we get to write an interpreter which looks like an interpreter for a strict functional language, but actually behaves lazily (by lifting haskell’s own lazy semantics into our interpreter).