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Add more CycleInDeclaration examples #333

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Add more CycleInDeclaration examples #333

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42 changes: 42 additions & 0 deletions errors/CycleInDeclaration.md
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Expand Up @@ -39,3 +39,45 @@ Note that cycles can also spring up in much less obvious ways, e.g. if you defin
## Notes

- If you are trying to implement a recursive parser in PureScript, then you might find this post on [recursive parsing in PureScript](https://github.com/Thimoteus/SandScript/wiki/2.-Parsing-recursively) to be helpful.

### Additional Examples

Mutually recursive functions:
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If we move the generic deriving example to be a link then we can change this header to be ### Mutual recursion and just focus on this mutual recursion example. I think that fits better with how other errors use the Notes section.

Also, what do you think of trimming the example a little bit to something like this?

isEven :: Int -> Boolean
isEven n 
  | n == 0 = true
  | otherwise = again (n - 1)

again :: Int -> Boolean
again = isEven

which also fails, producing this error:

  The value of isEven is undefined here, so this reference is not allowed.

while checking that expression isEven
  has type Int -> Boolean
in binding group isEven, again

```purs
{-
A very inneficient way to write:
zeroOrNine n = if n >= 9 then 9 else 0
-}
zeroOrNine :: Int -> Int
zeroOrNine 9 = 9
zeroOrNine n | n < 0 = 0
zeroOrNine n = again (n - 1)

again :: Int -> Int
again = zeroOrNine
{-
Fix compiler error by replacing the above line with either:
again n = zeroOrNine n
again = \n -> zeroOrNine n
-}
```
https://try.purescript.org/?gist=3e013405988ab9a58521412fddadca08

Type class instance for recursive data type:
```purs
data Chain a
= End a
| Link a (Chain a)

derive instance genericChain :: Generic (Chain a) _

-- This builds, but blows-up call stack and triggers a
-- "too much recursion error".
instance showChain :: Show a => Show (Chain a) where
show = genericShow
{-
Fix the issue by replacing the above line with:
show c = genericShow c
-}
```
https://try.purescript.org/?gist=32b78ce065d60427620cdd8d40777a1f
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Perhaps this could just be a link to the contents being added in #338?