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status](https://github.com/purescript/purescript-tailrec/workflows/CI/badge.svg?branch=master)](https://github.com/purescript/purescript-tailrec/actions?query=workflow%3ACI+branch%3Amaster)\n[![Pursuit](https://pursuit.purescript.org/packages/purescript-tailrec/badge)](https://pursuit.purescript.org/packages/purescript-tailrec)\n\nA type class which captures stack-safe monadic tail recursion.\n\n## Installation\n\n```\nspago install tailrec\n```\n\n## Usage\n\nThe PureScript compiler performs tail-call elimination for self-recursive functions, so that a function like\n\n```purescript\npow :: Int -\u003e Int -\u003e Int\npow n p = go { accum: 1, power: p }\n  where\n  go { accum: acc, power: 0 } = acc\n  go { accum: acc, power: p } = go { accum: acc * n, power: p - 1 }\n```\n\ngets compiled into an efficient `while` loop.\n\nHowever, we do not get the same benefit when using monadic recursion:\n\n```purescript\npowWriter :: Int -\u003e Int -\u003e Writer Product Unit\npowWriter n = go\n  where\n  go 0 = return unit\n  go m = do\n    tell n\n    go (m - 1)\n```\n\nHowever, we can refactor the original function to isolate the recursive function call:\n\n```purescript\npow :: Int -\u003e Int -\u003e Int\npow n p = tailRec go { accum: 1, power: p }\n  where\n  go :: _ -\u003e Step _ Int\n  go { accum: acc, power: 0 } = Done acc\n  go { accum: acc, power: p } = Loop { accum: acc * n, power: p - 1 }\n```\n\nwhere the `tailRec` function is defined in the `Control.Monad.Rec.Class` module, with type:\n\n```purescript\ntailRec :: forall a b. (a -\u003e Step a b) -\u003e a -\u003e b\n```\n\nIn 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.\n\nThis pattern can be generalized to several monad transformers from the `purescript-transformers` library using the following type class:\n\n```purescript\nclass Monad m \u003c= MonadRec m where\n  tailRecM :: forall a b. (a -\u003e m (Step a b)) -\u003e a -\u003e m b\n```\n\n## Documentation\n\nModule documentation is [published on Pursuit](http://pursuit.purescript.org/packages/purescript-tailrec).\n","funding_links":[],"categories":[],"sub_categories":[],"project_url":"https://awesome.ecosyste.ms/api/v1/projects/github.com%2Fpurescript%2Fpurescript-tailrec","html_url":"https://awesome.ecosyste.ms/projects/github.com%2Fpurescript%2Fpurescript-tailrec","lists_url":"https://awesome.ecosyste.ms/api/v1/projects/github.com%2Fpurescript%2Fpurescript-tailrec/lists"}