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Fix overloaded labels example #221

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Fix overloaded labels example #221

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c45d827
Fix overloaded labels example
djfo Aug 22, 2021
1f6adff
Include code snippet
djfo Aug 22, 2021
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23 changes: 2 additions & 21 deletions tutorial.md
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Expand Up @@ -5898,27 +5898,8 @@ extract :: IsLabel "id" t => t
extract = #id
```

```haskell
{-# LANGUAGE OverloadedLabels #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE ExistentialQuantification #-}

import GHC.Records (HasField(..)) -- Since base 4.10.0.0
import GHC.OverloadedLabels (IsLabel(..))

data S = MkS { foo :: Int }
data T x y z = forall b . MkT { foo :: y, bar :: b }

instance HasField x r a => IsLabel x (r -> a) where
fromLabel = getField

main :: IO ()
main = do
print (#foo (MkS 42))
print (#foo (MkT True False))
```
~~~~ {.haskell include="src/03-monad-transformers/overloaded_labels.hs"}
~~~~

This is used in more advanced libraries like [Selda] which do object relational
mapping between Haskell datatype fields and database columns.
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