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Update README.md #7

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2 changes: 1 addition & 1 deletion README.md
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Expand Up @@ -439,7 +439,7 @@ instance {-# OVERLAPPABLE #-}
TypeError (Text "First species counterpoint only have whole durations.")
=> ValidDur d
```
This makes use of GHC's [custom type errors](https://ghc.haskell.org/trac/ghc/wiki/Proposal/CustomTypeErrors) feature, with the correspoinding types available in the GHC.TypeLits module. We define the `ValidDur` type class (restricting its argument to a type of kind `Duration`) without any methods. The `Whole` duration is made an instance of `ValidDur`, expressing our intention to make whole durations valid. The overlappable instance for `ValidDur d` is selected only if the previous instance does not fit (i.e. the duration is not `Whole`): in this case, a custom type error is encountered and type-checking fails, displaying our custom error message.
This makes use of GHC's [custom type errors](https://ghc.haskell.org/trac/ghc/wiki/Proposal/CustomTypeErrors) feature, with the corresponding types available in the GHC.TypeLits module. We define the `ValidDur` type class (restricting its argument to a type of kind `Duration`) without any methods. The `Whole` duration is made an instance of `ValidDur`, expressing our intention to make whole durations valid. The overlappable instance for `ValidDur d` is selected only if the previous instance does not fit (i.e. the duration is not `Whole`): in this case, a custom type error is encountered and type-checking fails, displaying our custom error message.
* `ChordConstraints` and `ProgConstraints` are simply equal to the "constant" (nullary) type classes `InvalidChord` and `InvalidProg`, which are defined in a similar manner to `ValidDur`:

```haskell
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