This is a GHC plugin that enables imperative style loops, mutable variables, and early returns within any monadic do-block. The approach used is heavily inspired by the do block features of Lean 4 and this related paper.
To use it, add this package as a dependency and add
{-# OPTIONS_GHC -fplugin Donuts #-} to the top of a Haskell file and
import the Donuts module.
Use Mut to bind a mutable variable and := to assign it to a new value. The
mutability is scoped to the do block it is defined in as well as any nested
loop or when bodies, if statements, and case statements within that block.
mutability :: Int
mutability = runIdentity $ do
let Mut x = 5
x := x * 2
pure x
mutBind :: Maybe Int
mutBind = do
Mut x <- Just 22
x := 1
guard (odd x)
pure x
-- Strict and lazy annotations are respected, as well as the -XStrict extension.
-- If strict, a mutable variable is evaluated to WHNF upon binding and re-assignment.
-- Mutable variables are lazy by default.
mutStrict :: Int
mutStrict = runIdentity $ do
let Mut !x = computation
x := x * 2
pure xearlyReturn lets you short circuit a monadic computation, returning a given value.
secretNum :: IO Int
secretNum = do
inp <- getLine
when (inp == "secret") (earlyReturn 42)
putStrLn "Invalid password"
pure 0forL provides imperative style for-loops over any Foldable container. There
is also whileL for while-loops and repeatL for indefinitely repeating loops.
Within loop bodies, continueL and breakL provide the functionality
of their counterparts from imperative languages.
skipEven :: IO ()
skipEven = do
let i = 20 :: Int
forL [1..i] $ \j -> do
when (even j) continueL
print j
countdown :: IO ()
countdown = do
let Mut x = 5
whileL (x > 0) $ do
print x
x := x - 1
loopBreak :: IO ()
loopBreak = do
let Mut strings = []
repeatL $ do
inp <- getLine
when (inp == "stop") breakL
strings := inp : strings
print strings