chainl1 parser example in PureScript

Main.purs

-- attempt to port `chainl1` example from
-- https://hackage.haskell.org/package/parsec-3.1.14.0/docs/Text-Parsec-Combinator.html#v:chainl1

module Main where

import Prelude hiding (between)
import Text.Parsing.StringParser
import Text.Parsing.StringParser.Combinators
import Text.Parsing.StringParser.CodePoints
import Control.Alt
import Partial.Unsafe
import Data.Maybe
import Data.Int as I
import Control.Lazy

parens :: forall a. Parser a -> Parser a
parens = between (string "(") (string ")")

{- the following all have errors like:
    The value of term is undefined here, so this reference is not allowed.
    PureScript(CycleInDeclaration)
-}
expr = defer (\_ -> term) `chainl1` addop
term    = defer (\_ -> factor) `chainl1` mulop
factor  = (parens $ defer (\_ -> expr)) <|> int

int :: Parser Int
int =
  toInt <$> regex "[0-9]+"
  where
    toInt :: String -> Int
    toInt = unsafePartial fromJust <<< I.fromString 

skip :: forall a. Parser a -> Parser Unit
skip = void

mulop =
    do
        skip $ string "*"
        pure (*)
    <|> do
        skip $ string "/"
        pure (/)

addop =
    do
        skip $ string "+"
        pure (+)
    <|> do
        skip $ string "-"
        pure (-)