Thanks. It was nice to read. I was reminded of "The Monad Reader Issue 6" as it explains how to "tie the knot" when creating labels: http://www.haskell.org/wikiupload/1/14/TMR-Issue6.pdf

The Russell uses an assembly language DSL as a motivating example and explains how either Haskell98 or mdo can be used to tie the knot, with mdo giving a nicer implementation. The result allows labels to be defined where they are used instead of requiring the user to instantiate them and then associate them with a place in the program. This provides a nice abstraction for building your assembler macros.

For example, in your code we see:

\begin{code}

testProgram :: Asm ()
testProgram = do
  begin <- label
  loop  <- label
  begin -: do
    i1 10 20
    i2

    i3 5

  loop -: do
    i4

    goto loop

\end{code}

The corresponding code using the TMR trick might look like this (I've removed the nested indentation):

\begin{code}

testProgram :: Asm ()
testProgram = mdo
  begin <- label
  i1 10 20
  i2

  i3 5

  loop  <- label
  i4

  goto loop

\end{code}

Other examples demonstrate that labels can be referenced before the line that creates them due to the way mdo works.

Jason