Problem Statement

Subleq is an example of a One-Instruction Set Computer (OISC). It is named after its only instruction, which is SUbtract and Branch if Less than or EQual to zero.
Your task is to create an interpreter which emulates a SUBLEQ machine. The machine's memory consists of an array of signed integers.   These integers may be interpreted in three ways: Any reasonable word size that accommodates all three of the above uses is fine. The program should load the initial contents of the emulated machine's memory, set the instruction pointer to the first address (which is defined to be address 0), and begin emulating the machine, which works as follows: Your solution may initialize the emulated machine's memory in any convenient manner, but if you accept it as input, it should be a separate input stream from the one fed to the emulated machine once it is running. And if fed as text input, it should be in the form of raw subleq "machine code" - whitespace-separated decimal numbers, with no symbolic names or other assembly-level extensions, to be loaded into memory starting at address   0   (zero). For purposes of this task, show the output of your solution when fed the below   "Hello, world!"   program. As written, this example assumes ASCII or a superset of it, such as any of the Latin-N character sets or Unicode;   you may translate the numbers representing characters (starting with 72=ASCII 'H') into another character set if your implementation runs in a non-ASCII-compatible environment. If 0 is not an appropriate terminator in your character set, the program logic will need some adjustment as well. The above "machine code" corresponds to something like this in a hypothetical assembler language for a signed 8-bit version of the machine:

Let's start with the solution:

{-# LANGUAGE FlexibleContexts #-}
import Control.Monad.State
import Data.Char (chr, ord)
import Data.IntMap

subleq = loop 0
    where
      loop ip =
          when (ip >= 0) $
          do m0 <- gets (! ip)
             m1 <- gets (! (ip + 1))
             if m0 < 0
                then do modify . insert m1 ch . ord =<< liftIO getChar
                        loop (ip + 3)
                else if m1 < 0
                        then do liftIO . putChar . chr =<< gets (! m0)
                                loop (ip + 3)
                        else do v <- (-) <$> gets (! m1) <*> gets (! m0)
                                modify $ insert m1 v
                                if v <= 0
                                   then loop =<< gets (! (ip + 2))
                                   else loop (ip + 3)

main = evalStateT subleq helloWorld
    where
      helloWorld =
          fromList $
          zip [0..]
              [15, 17, -1, 17, -1, -1, 16, 1, -1, 16, 3, -1, 15, 15, 0, 0, -1, 72, 101, 108, 108, 111, 32, 119, 111, 114, 108, 100, 33, 10, 0]