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Note annotation requirements in let and where #350

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Note annotation requirements in let and where #350

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24 changes: 24 additions & 0 deletions language/Syntax.md
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Expand Up @@ -527,6 +527,30 @@ factorial = go 1
go acc n = go (acc * n) (n - 1)
```

### Caveat for generalized functions

PureScript does not implicity generalize functions defined within `let` or `where` blocks. See ["Let Should not be Generalised"](https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/tldi10-vytiniotis.pdf) for the motivation behind this design choice.

So while the `asArray` annotations in the following snippet may seem unnecessary, they are required for compilation.
```purs
arrayPairIn :: forall a b. a -> b -> Tuple (Array a) (Array b)
arrayPairIn x y =
let
-- This annotation is required
asArray :: forall c. c -> Array c
asArray z = [z]
in
Tuple (asArray x) (asArray y)

arrayPairWhere :: forall a b. a -> b -> Tuple (Array a) (Array b)
arrayPairWhere x y =
Tuple (asArray x) (asArray y)
where
-- This annotation is required
asArray :: forall c. c -> Array c
asArray z = [z]
```

## Indentation in binding blocks

Indentation of a binding's body is significant. If defining multiple bindings, as in a let-in block, each binding must have the same level of indentation. The body of the binding's definition, then, must be further indented. To illustrate:
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