H2JVM - A Haskell Library for writing JVM Bytecode

Looks good and plainly written, I’ll bookmark it for next time I do something with the JVM

Browsed around and reviewed a little, have a few random suggestions

You can avoid the possibility of an UnmarkedLabel error using a MonadFix instance and rec or mdo

emitNewLabel = do
  label <- newLabel
  emit $ JVM.Label label
  pure label



mdo
  emit $ JVM.IfICmp (IfGt trueLabel)
  emit JVM.IConst0
  emit $ Goto endLabel
  emit JVM.IConst1
  trueLabel <- emitNewLabel
  emit JVM.IConst1
  endLabel <- emitNewLabel
  pure ()

I’d rather use rec to spell out the scope, but this case is better with mdo because the labels are used throughout

In CodeState instead of storing a list in reverse order you can use a DList Instruction

Code that uses IfCond is repetitive because it’s denormalised

data IfCond label = IfEq label | IfNe label | IfLt label | IfGe label | IfGt label | IfLe label

Instead you can factor:

1 x +1 x +1 x +1 x +1 x +1 x = (1+1+1+1+1+1)x = 6 x

data If label = If Cond label
data Cond = Eq | Ne | Lt | Ge | Gt | Le

Can also keep the current patterns with synonyms like pattern IfEq label = If Eq label and a {-# COMPLETE #-} pragma

In clauses of Pretty instances like this

pretty (InvokeSpecial c n d) =
  "invokespecial" <+> pretty c <> "." <> pretty n <> pretty d

I like to use ViewPatterns (or a synonym) so the shape of the output is less cluttered by calls to pretty, and it’s only called once if used more than once

pretty (InvokeSpecial (pretty -> c) (pretty -> n) (pretty -> d)) =
  "invokespecial" <+> c <> "." <> n <> d

I dunno how “pretty” you actually want for the pretty-printing to be, but I find that Pretty instances in general tend to overuse the explicit non-breaking horizontal (hcat, hsep, <>, <+>) and vertical (vcat, vsep) layouts without enough grouping, and should rather use the grouped combinators that allow breaks between all (cat, sep, surround softline', surround softline) or between any (fillCat, fillSep)