Plain interpreter. No, compiler.

I was a bit ill during the weekend, so instead of going to seashore played with Haskell.
I shot for an interpreter of Plain in Haskell, but ended with a compiler from Plain to a STM-like intermediate language (promises and atomic execution - let's call it PAL) and an interpreter for PAL in Haskell (let's call it PVM). This looks promising (pun intended).

What I need to do before going further is:

1. Fix implementation of variables in PVM (this is currently a strange hybrid of de Bruijn indices and dynamic scoping).
   
2. Express PAL statements in monadic form to simplify writing compiler(s).
   
3. Express PVM in monadic form to simplify experimenting with it.
   
4. Externalize scheduling (probably in a way of A Language-Based Approach to Unifying Events and Threads). Amusingly, an atomic execution could be elegantly expressed as a strict node in the trace tree.
   
5. Explore possibilities for a type system for PAL (A Functional Abstraction of Typed Contexts?).