tailrec

A type class which captures stack-safe monadic tail recursion.

spago install tailrec

The PureScript compiler performs tail-call elimination for self-recursive functions, so that a function like

pow :: Int -> Int -> Int
pow n p = go { accum: 1, power: p }
  where
  go { accum: acc, power: 0 } = acc
  go { accum: acc, power: p } = go { accum: acc * n, power: p - 1 }

gets compiled into an efficient while loop.

However, we do not get the same benefit when using monadic recursion:

powWriter :: Int -> Int -> Writer Product Unit
powWriter n = go
  where
  go 0 = return unit
  go m = do
    tell n
    go (m - 1)

However, we can refactor the original function to isolate the recursive function call:

pow :: Int -> Int -> Int
pow n p = tailRec go { accum: 1, power: p }
  where
  go :: _ -> Step _ Int
  go { accum: acc, power: 0 } = Done acc
  go { accum: acc, power: p } = Loop { accum: acc * n, power: p - 1 }

where the tailRec function is defined in the Control.Monad.Rec.Class module, with type:

tailRec :: forall a b. (a -> Step a b) -> a -> b

In the body of the loop, instead of calling the go function recursively, we return a value using the Loop constructor. To break from the loop, we use the Done constructor.

This pattern can be generalized to several monad transformers from the purescript-transformers library using the following type class:

class Monad m <= MonadRec m where
  tailRecM :: forall a b. (a -> m (Step a b)) -> a -> m b

Module documentation is published on Pursuit.