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Non-trivial type synonyms

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To avoid Miles/Km , feet/metres goofs:
To avoid Miles/Km , feet/metres goofs:
-
rather than
+
Rather than
<haskell>
<haskell>
type Miles = Int
type Miles = Int
Line 24: Line 24:
The functions toMiles and fromMiles is already effectively id !
The functions toMiles and fromMiles is already effectively id !
-
A "newtype" definition just generates a new type whose implementation is the
+
A <hask>newtype</hask> definition just generates a new type whose implementation is the same as the old one.
-
same as the old one.
+
-
The constructor (and possibly selector) really does nothing except coerce
+
The constructor (and possibly selector) really does nothing except coerce between the new type and the old one.
-
between the new type and the old one.
+
-
So as <hask>undefined</hask> is Haskells notation for bottom
+
So as <hask>undefined</hask> is Haskell's notation for bottom
<haskell>
<haskell>
toMiles undefined === undefined
toMiles undefined === undefined
</haskell>
</haskell>
-
which isn't the case if we use a "data" definition.
+
which isn't the case if we use a <hask>data</hask> definition.
Line 44: Line 42:
{- or just use Miles, though this is harder to excise later -}
{- or just use Miles, though this is harder to excise later -}
</haskell>
</haskell>
-
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]].
+
For tracking more complex units, you may be able to get some of the way by using [[phantom type]]s or creative use of [[functional dependencies]].
==See also==
==See also==
-
* For an example toy implementation using [[Phantom types]] & [[Functional dependencies]], see [[Dimensionalized numbers]]
+
* For an example toy implementation using [[phantom type]]s & [[functional dependencies]], see [[dimensionalized numbers]]
* [[Wrapper types]].
* [[Wrapper types]].
[[Category:Idioms]]
[[Category:Idioms]]

Current revision

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.


2 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 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.


3 See also

Retrieved from "http://www.haskell.org/haskellwiki/Non-trivial_type_synonyms"

Category: Idioms