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Merged
merged 2 commits into from
Oct 13, 2024
Merged

Document GHC-08838 #543

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52dc88a
Document GHC-08838
StillerHarpo Oct 13, 2024
b08994a
Update message-index/messages/GHC-08838/index.md
BinderDavid Oct 13, 2024
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4 changes: 4 additions & 0 deletions message-index/messages/GHC-08838/WrongDoBind/after/WrongDoBind.hs
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module WrongDoBind where

doubleReturn :: forall m. Monad m => m Int
doubleReturn = return 10
6 changes: 6 additions & 0 deletions message-index/messages/GHC-08838/WrongDoBind/before/WrongDoBind.hs
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module WrongDoBind where

doubleReturn :: forall m. Monad m => m Int
doubleReturn = do
return (return 10 :: m Int)
return 10
16 changes: 16 additions & 0 deletions message-index/messages/GHC-08838/WrongDoBind/index.md
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---
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Suggested change
---
---

title: Double return
---

In this example, there is a nested return whose result is not bound to a variable. Therefore the computation `return 10` is not used and can be removed.

```
messages/GHC-08838/WrongDoBind/before/WrongDoBind.hs:5:4: warning: [GHC-08838] [-Wwrong-do-bind]
A do-notation statement discarded a result of type ‘m Int’
Suggested fix:
Suppress this warning by saying ‘_ <- return (return 10 :: m Int)’
|
5 | return (return 10 :: m Int)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
9 changes: 9 additions & 0 deletions message-index/messages/GHC-08838/index.md
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---
title: Wrong do bind
summary: Returning a monadic computation in a do block and not binding it will not run it
severity: warning
introduced: 9.6.1
---
When using a monadic computation in a do block one should either run it
directly or bind the result of the computation to a variable. If neither
is done the monadic computation is never run and can be removed.