# purescript-tailrec

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

## Installation

```
spago install tailrec
```

## Usage

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
```

## Documentation

Module documentation is published on Pursuit.