# What to do if you have a Non Exhaustive pattern match

**URL:** <https://discourse.elm-lang.org/t/what-to-do-if-you-have-a-non-exhaustive-pattern-match/8146>\
**Category:** Learn\
**Created:** [February 8, 2022, 4:56pm UTC](https://discourse.elm-lang.org/t/what-to-do-if-you-have-a-non-exhaustive-pattern-match/8146 "2022-02-08T16:56:26Z")\
**Posts on this page:** 1\
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**Author:** ![eike](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.elm-lang.org/eike/32/749_2.png) [@eike](https://discourse.elm-lang.org/u/eike)\
**Post date:** [February 9, 2022, 6:14pm UTC](https://discourse.elm-lang.org/t/what-to-do-if-you-have-a-non-exhaustive-pattern-match/8146/5 "2022-02-09T18:14:48Z")

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“No runtime errors” is a very high priority for Elm. This means that you are not allowed to create functions that can fail. (Unless you’re implementing a core function like `modBy 0`.) The unsatisfying the solution of choosing a default value is probably your best choice. What I would do is:

1. Use `Debug.todo` in your code while developing to mark the impossible branches.
2. Write exhaustive tests and make sure you never run into those errors.
3. Replace `Debug.todo` by a default value.

(You might consider writing a custom wrapper around `Debug.todo` that already takes a value of the correct type so that you only need to change the definition of your custom wrapper before finalizing the module instead of replacing every occurrence of `Debug.todo`.)

Unfortunately, there are still two problems:

1. Having to use different code for development and production builds is annoying.
2. If you are tasked with producing a value of an unknown type (say you have a function `MyTree a -> a` and you know that the tree will always contain a value to return but you can’t prove it to the compiler), you’re out of luck and your “infinite recursion” solution is basically your only choice.

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