edit- A monad for rewriting things.

Copyright(c) Varun Gandhi 2018
LicenseBSD-style (see the file LICENSE)
Safe HaskellSafe




This is a short (?) tutorial describing how you can use the Edit module to help you with writing dataflow analysis code for a compiler. The example is a bit artificial for the sake of relative conciseness -- if you have a better suggestion, or find any mistakes, please let me know on the Github issue tracker.



Get a fixed point from applying a sequence of transformations.

import Data.Edit (Edit, edits, polish, (>=>))

mkAwesome1 :: Foo -> Maybe Foo
mkAwesomeN :: Foo -> Maybe Foo

mkAwesomeAny :: Foo -> Edit Foo
  = foldr (\f acc -> acc >=> (f `edits`)) pure
    [mkAwesome1, ..., mkAwesomeN]

mkAsAwesomeAsPossible :: Foo -> Foo
mkAsAwesomeAsPossible = polish mkAwesomeAny

Transform a recursive data structure, keeping track of whether it was changed or not, and feed the result to some high-level dataflow analysis function.

import DataFlowLibrary
import PlatedLibrary
import Data.Edit (Edit, edits, toMaybe)

instance FancyPlate Foo where ...

mkAwesome :: Foo -> Maybe Foo
mkAwesome = ...

mkTotallyAwesome :: Foo -> Edit Foo
mkTotallyAwesome = transformM (mkAwesome `edits`)

dataFlowAnalysis = dataFlowLibFn (toMaybe . mkTotallyAwesome)


The examples here use the Uniplate and Containers libraries. If you want to follow along as we proceed, you will want to supply the package flag tutorial and maybe read the docs in your browser.

If you're testing inside a cabal sandbox, this can be done using

cabal configure --flags="tutorial"
cabal build
cabal haddock

If you're using stack, the same can be done using:

stack build --flag=edit:tutorial
stack haddock --flag=edit:tutorial --open edit


Let's define a toy language L with Ints and addition.

newtype Ident = Ident String
  deriving (Show, Eq)

data Expr
  = Val Int
  | Var Ident
  | Add Expr Expr
  deriving (Show, Eq)

Q. How would you implement constant folding for the Expr type?

  1. Write the recursion by hand. While this is easy enough to do since Expr only has a few constructors, this isn't very practical when you have lots of constructors. The exact point where you recognize that this is a recursive descent into unmaintainability depends on your personal boilerplate threshold.
  2. Use recursion schemes and get lost in the unfathomable type errors (I'm half-joking). While this is a reasonable approach, we're not going to follow this here.
  3. Use a generics library. For simplicity, we'll be using Uniplate here. The particular functions that are relevant at the moment are rewrite and transform. Let's use rewrite.
{-# LANGUAGE DeriveDataTypeable #-}

import Data.Data
import Data.Generics.Uniplate.Data

newtype Ident = Ident String
  deriving (Show, Eq, Typeable, Data)

data Expr
  = Val Int
  | Var Ident
  | Add Expr Expr
  deriving (Show, Eq, Typeable, Data)

constFold :: Expr -> Expr
constFold e = rewrite go e
    go (Add (Val i) (Val j)) = Just (Val (i + j))
    go _ = Nothing

Test that the implementation works as expected.

>>> two = Add (Val 1) (Val 1)
>>> four = Add (Val 2) (Val 2)
>>> constFold (Add two four)
Val 6
>>> constFold (Add (Var "x") two)
Add (Var "x") (Val 2)

Let's say we add assignment statements to the language and write a function to do constant propagation. First we add a substitute function.

import Data.Map (Map)
import qualified Data.Map as Map

newtype Ident = Ident String
  deriving (Eq, Ord, Show, Typeable, Data)

substitute :: Map Ident Int -> Expr -> Expr
substitute m e = rewrite go e
    go (Var x) = Val <$> Map.lookup x m
    go _ = Nothing

Let's test this out.

>>> x = Var (Ident "x")
>>> quadrupleX = Add x (Add x (Add x x))
>>> m1 = Map.fromList [(Ident "x", 5)]
>>> substitute m1 quadrupleX
Add (Val 5) (Add (Val 5) (Add (Val 5) (Val 5)))

Finally putting all the pieces together. We can use the polish function to find the fixed point, which (in this case) is a fancy way of saying that we keep iterating until we have a Clean (unchanged) value.

constFoldAndPropPass :: [Stmt] -> [Stmt]
constFoldAndPropPass = polish (constFoldPass >=> constPropPass)

We're not done yet though! We still need to check that this works :P.

>>> [w, x, y] = map Ident ["w", "x", "y"]
>>> s1 = w := Add (Val 1) (Val 2)
>>> s2 = x := Add (Var w) (Var w)
>>> s3 = y := Add (Var w) (Add (Val 1) (Var x))
>>> s4 = x := Add (Var y) (Var y)
>>> s5 = y := Add (Var w) (Var x)
>>> constFoldAndPropPass [s1, s2, s3, s4, s5]
[Ident "w" := Val 3,Ident "x" := Val 6,Ident "y" := Val 10,Ident "x" := Val 20,Ident "y" := Val 23]

Yup, it works! For fun, let's see the transformation process in action. We can do this using the iterations function.

>>> pprint = putStr . unlines . map (unlines . map show)
>>> pprint $ iterations (constFoldPass >=> constPropPass) [s1, s2, s3, s4, s5]

The output shows the full history, with the final result that we obtained earlier at the end.

Ident "w" := Add (Val 1) (Val 2)
Ident "x" := Add (Var (Ident "w")) (Var (Ident "w"))
Ident "y" := Add (Var (Ident "w")) (Add (Val 1) (Var (Ident "x")))
Ident "x" := Add (Var (Ident "y")) (Var (Ident "y"))
Ident "y" := Add (Var (Ident "w")) (Var (Ident "x"))

Ident "w" := Val 3
Ident "x" := Add (Val 3) (Val 3)
Ident "y" := Add (Val 3) (Add (Val 1) (Var (Ident "x")))
Ident "x" := Add (Var (Ident "y")) (Var (Ident "y"))
Ident "y" := Add (Val 3) (Var (Ident "x"))

Ident "w" := Val 3
Ident "x" := Val 6
Ident "y" := Add (Val 3) (Add (Val 1) (Val 6))
Ident "x" := Add (Var (Ident "y")) (Var (Ident "y"))
Ident "y" := Add (Val 3) (Var (Ident "x"))

Ident "w" := Val 3
Ident "x" := Val 6
Ident "y" := Val 10
Ident "x" := Add (Val 10) (Val 10)
Ident "y" := Add (Val 3) (Var (Ident "x"))

Ident "w" := Val 3
Ident "x" := Val 6
Ident "y" := Val 10
Ident "x" := Val 20
Ident "y" := Add (Val 3) (Val 20)

Ident "w" := Val 3
Ident "x" := Val 6
Ident "y" := Val 10
Ident "x" := Val 20
Ident "y" := Val 23