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Guard Limitations in let #317

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Guard Limitations in let #317

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Guard Limitations in `let`
milesfrain May 17, 2020
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milesfrain Aug 22, 2020
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lowercase guard
milesfrain Aug 23, 2020
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30 changes: 30 additions & 0 deletions language/Pattern-Matching.md
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Expand Up @@ -176,6 +176,36 @@ compare _ _ = EQ

(The name `otherwise` is a synonym for `true` commonly used in guards.)

Guard Limitations in `let`
----------------------------

Guards are [currently not supported](purescript/purescript#3200) with patterns other than simple identifiers in `let` expressions. For example, this does not compile:
```purs
-- This doesn't work
f1 :: Int
f1 =
let
(Tuple a b)
| false = Tuple 1 2
| otherwise = Tuple 3 4
in
a
```

A workaround is to separate the constructor pattern from the guard:
```purs
f2 :: Int
f2 =
let
t
| false = Tuple 1 2
| otherwise = Tuple 3 4

Tuple a b = t
in
a
```

Pattern Guards
--------------

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