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).