Examples of monadic effects inside a rewrite function in Hoopl?

Question

The type of (forward) rewriting functions in Hoopl is given by the mkFRewrite function:

mkFRewrite :: (FuelMonad m) => 
   (forall e x.
      n e x
      -> f
      -> m (Maybe (hoopl-3.8.6.1:Compiler.Hoopl.Dataflow.Graph n e x)))
   -> FwdRewrite m n f

The m type implies that I can use monadic effects while rewriting. The paper "Hoopl: A Modular, Reusable Library for Dataflow Analysis and Transformation" says the same in Section 4.3, "The rewrite function and the client's monad."

Can anyone give me an example of a rewrite function that has non-Hoopl monadic effects embedded inside it? For example, a rewriter that uses a State monad or does some IO.

Answer 1

This should be pretty simple, just chase the types.

You want a value of FwdRewrite m n f with a custom value of m, so you can pass it to the following function:

analyzeAndRewriteFwd ::
  forall m n f e x entries.
    (CheckpointMonad m,
     NonLocal n,
     LabelsPtr entries) =>
  FwdPass m n f ->
  MaybeC e entries ->
  Graph n e x ->
  Fact e f ->
  m (Graph n e x, FactBase f, MaybeO x f)

So the only constraint on m you have is that it is a CheckpointMonad; then when you run the pass you'll get the final monadic value which you can run yourself.

In fact, GHC's Hoopl passes use with m as a SimplUniqMonad, so we can get fresh labels while we're operating on the graph.

{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE TypeFamilies #-}

import Compiler.Hoopl
import Control.Monad.State

type StateFuel s a = CheckingFuelMonad (State s) a

instance CheckpointMonad (State s) where
    type Checkpoint (State s) = s
    checkpoint = get
    restart = put