from-1.0.0.1: README.md

# from

Note that this package is a part of the "project" that includes from and from-string. For the general rationale behind this idea, see `README.md` under the project root.

This Haskell library package provides the `From` and `TryFrom` typeclasses.

The `From` typeclass provides `from :: a -> b`. It is an interface for types that can be converted from/to each other.

The `TryFrom` typeclass provides `tryFrom :: a -> Maybe b`. It is an interface for types that can be converted from/to each other, with the possibility of a failure.

This package has **1** dependency: base.

## Instances

It is difficult and often impossible to "selectively" import/export typeclass instances. Therefore, we need to be careful about what instances to provide by default.

There are base functions, some of them even in `Prelude`, that basically serve as a less general version of `from`. Examples include `fromIntegral` and `fromEnum`. However, naively defining

- `instance (Integral a, Num b) => From a b` and
- `instance (Enum a) => From a Int`

will quickly lead us to trouble:

```
    • Overlapping instances for From Int16 Int
        arising from a use of ‘from’
      Matching instances:
        instance (Integral a, Num b) => From a b
        instance Enum a => From a Int
      ...
```

Therefore, this package does not provide such "contextual" instances.

Instead, concrete instances are provided. For example, all pairs (excluding self to self) of these integral types are instantiated using `fromIntegral`:

- `Int`
- `Integer`
- `Int8`
- `Int16`
- `Int32`
- `Int64`
- `Word8`
- `Word16`
- `Word32`
- `Word64`

Each type can be both source and destination of `from`. Additional types are instantiated as destination-only:

- `Float`
- `Double`