Non-trivial type synonyms

1 Make the type system work for you

To avoid Miles/Km , feet/metres goofs:

Rather than

type Miles = Int

use

newtype Miles = Miles Int

together with

toMiles :: Int -> Miles
	fromMiles :: Miles -> Int

these can be redefined to be "id" later, after the code is stablized.

No need to change things : The functions toMiles and fromMiles is already effectively id !

newtype

definition just generates a new type whose implementation is the same as the old one.

The constructor (and possibly selector) really does nothing except coerce between the new type and the old one.

So as

undefined

is Haskell's notation for bottom

toMiles undefined === undefined

which isn't the case if we use a

data

definition.

Structures might be a bit more convenient here.

newtype Miles = Miles { fromMiles :: Int }
  toMiles = Miles
      {- or just use Miles, though this is harder to excise later -}

For tracking more complex units, you may be able to get some of the way by using phantom types or creative use of functional dependencies.