Non-trivial type synonyms

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.

What do you mean by "redefined to be 'id'"? (New to Haskell, though I like it a lot.) Instinctively I dislike the idea of changing things after the code is "stabilized", because code never stabilizes unless because people are afraid to touch it because it's too fragile. I really like the idea of making the type system track units like that... Has anybody worked out the details of a system that would use classes and inheritance to automate the tracking of complex units like Miles/Hour? If I do, I'll probably post it here. --User:DanielKnapp

Alternate Miles etc. definitions

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

A "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 Haskells notation for bottom

	toMiles undefined === undefined

which isn't the case if we use a "data" definition.

One possibly valid reason for removing the constructors and selector may be that they clutters the code too much. (I think restricted type synonyms can sometimes fix this better)

-- StefanLjungstrand

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 PhantomTypes or creative use of FunDeps.

See also

• For an example toy implementation using Phantom types & Functional dependencies, see Dimensionalized numbers
• Wrapper types.

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CategoryIdiom