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Expand on CycleInDeclaration and fixes #241

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Feb 23, 2019
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Expand on CycleInDeclaration and fixes #241

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5 changes: 4 additions & 1 deletion errors/CycleInDeclaration.md
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Expand Up @@ -18,7 +18,7 @@ or to contribute content related to this error.

## Cause

This error occurs when a value refers to itself, or another value in the same binding group, at a point where such a reference would be unavailable due to _strict evaluation_.
This error occurs when a value refers to itself, or another value in the same binding group, at a point where such a reference would be unavailable because of _strict evaluation_.

In the example above, it would be incorrect to generate the JavaScript

Expand All @@ -28,9 +28,12 @@ var x = x;

since then `x` would be `undefined`.

Note that cycles can also spring up in much less obvious ways, e.g. if you define one typeclass member using another.

## Fix

- Consider the problem again, keeping PureScript's strict evaluation in mind. How would you solve this problem in JavaScript, for example?
- Sometimes you can break the cycle by strategically using eta expansion (`\x -> f x` instead of `f`) or using do-notation, which will introduce a function to the same effect because of `bind`.
- You might be able to make use of the `Control.Lazy` module, and the [`fix`](https://pursuit.purescript.org/packages/purescript-control/4.1.0/docs/Control.Lazy#v:fix) function in particular.

## Notes
Expand Down