# Recusion patterns
**URL:** https://discourse.elm-lang.org/t/recusion-patterns/4867
**Category:** Learn
**Created:** [December 22, 2019, 3:47pm UTC](https://discourse.elm-lang.org/t/recusion-patterns/4867 "2019-12-22T15:47:16Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@simonh1000](https://discourse.elm-lang.org/u/simonh1000)
#### Post date: [December 22, 2019, 3:47pm UTC](https://discourse.elm-lang.org/t/recusion-patterns/4867/1 "2019-12-22T15:47:16Z")
```auto
runCodeInner : ProgState -> ProgState
runCodeInner step =
case step of
Running state ->
let
tmp =
doStep state
in
runCodeInner tmp
--Running state ->
-- state
-- |> doStep
-- |> runCodeInner
--
_ ->
step
```
This snippet is from my Advent of Code work. My original code was the commented out part and I was getting a stack overflow. I had reached a question where there were warnings that things could take a while on slow computers, so I had a feeling that the issue was my recursion pattern.
I’m not an expert in ‘tail recursion’ but I thought the original code was tail recursive? It seems not, but that my alternative is and completed quickly. Something to do with evaluating it to `tmp` then I guess?
There’s clearly something I can learn here about how recursion works in Elm and would welcome feedback on that.
---
### Author:  [@folkertdev](https://discourse.elm-lang.org/u/folkertdev)
#### Post date: [December 22, 2019, 3:53pm UTC](https://discourse.elm-lang.org/t/recusion-patterns/4867/2 "2019-12-22T15:53:16Z")
This is a change with the desugaring of `|>` in 0.19. Previously, this happened early on, now it happens after tail-recursion is recognized.
Tail-recursive statements now have to look like `f x y z`. The seemingly equivalent `z |> f x y` is not recognized as tail-recursive.
---
### Author:  [@jfmengels](https://discourse.elm-lang.org/u/jfmengels)
#### Post date: [December 22, 2019, 3:55pm UTC](https://discourse.elm-lang.org/t/recusion-patterns/4867/3 "2019-12-22T15:55:07Z")
Just like @folkertdev said. An issue was raised in the compiler, which you can read here: [https://github.com/elm/compiler/issues/1770](https://github.com/elm/compiler/issues/1770)
---
### Author:  [@simonh1000](https://discourse.elm-lang.org/u/simonh1000)
#### Post date: [December 22, 2019, 4:38pm UTC](https://discourse.elm-lang.org/t/recusion-patterns/4867/4 "2019-12-22T16:38:32Z")
Yikes - this feels like a real gotcha at the heart of Elm. Thanks for pointing me to the issue
---
### Author:  [@system](https://discourse.elm-lang.org/u/system)
#### Post date: [January 1, 2020, 4:38pm UTC](https://discourse.elm-lang.org/t/recusion-patterns/4867/5 "2020-01-01T16:38:42Z")
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