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[Chapter 5] Add a note on Unit #460

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Nov 11, 2023
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[Chapter 5] Add a note on Unit #460

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Expand Up @@ -340,6 +340,14 @@ Just like `pure`, we can apply the `guard` function in PSCi to understand how it
forall (m :: Type -> Type). Alternative m => Boolean -> m Unit
```

> The `Unit` type represents values with no computational content — the absence of a concrete meaningful value.
>
> We often use `Unit` "wrapped" in a type constructor as the return type of a computation where we only care about the _effects_ of the computation (or a "shape" of the result) and not some concrete value.
>
> For example, the `main` function has the type `Effect Unit`. Main is an entry point to the project — we don't call it directly.
>
> We'll explain what `m` in the type signature means in Chapter 6.

In our case, we can assume that PSCi reported the following type:

```haskell
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