# cast

 LTS Haskell 16.16: 0.1.0.2 Stackage Nightly 2020-09-29: 0.1.0.2 Latest on Hackage: 0.1.0.2

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#### Module documentation for 0.1.0.2

This version can be pinned in stack with:cast-0.1.0.2@sha256:c8f6b8f9332c685cde9df90ebc0b1a7598dfa85fc8377b717ea05a92bb2fd087,877
• Pattern

This pattern allows to incapsulate convert from one type of object for another.

## Example

Suppose you want convert different speed units to meter per second:

{-# LANGUAGE MultiParamTypeClasses #-}

import Pattern.Cast

newtype MeterPerSecond   = MeterPerSecond Float
deriving (Ord, Eq)
newtype KilometerPerHour = KilometerPerHour Float
newtype MilesPerHour     = MilesPerHour Float

instance Cast KilometerPerHour MeterPerSecond where
cast (KilometerPerHour v) = MeterPerSecond (0.277778 * v)

instance Cast MilesPerHour MeterPerSecond where
cast (MilesPerHour v) = MeterPerSecond (0.44704 * v)

As you see, you have to use MultiParamTypeClasses language extension.

Then in every place you can just call one function cast:

> cast (KilometerPerHour 100) :: MeterPerSecond
MeterPerSecond 27.7778
> cast (MilesPerHour 100) :: MeterPerSecond
MeterPerSecond 44.704

You can type your functions more abstractly. Letâ€™s look at this synthetic example:

type Second = Float
type Meter  = Float

data Aircraft = Aircraft { distance :: Meter
, time     :: Second
}

instance Cast Aircraft MeterPerSecond where
cast (Aircraft d t) = MeterPerSecond (d / t)

Then you can use Cast in type of your fuction like this (FlexibleContexts extension has to be used):

{-# LANGUAGE FlexibleContexts #-}

slowerThenSound :: Cast a MeterPerSecond => a -> Bool
slowerThenSound x = cast x < MeterPerSecond 340.29

And this fuction can be used with every type that can be converted in MeterPerSecond:

> slowerThenSound \$ MeterPerSecond 200
True
> slowerThenSound \$ KilometerPerHour 1000
True
> slowerThenSound \$ Aircraft 1200 3
False
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