Difference between revisions of "Non-trivial type synonyms"
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To avoid Miles/Km , feet/metres goofs: |
To avoid Miles/Km , feet/metres goofs: |
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| − | + | Rather than |
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<haskell> |
<haskell> |
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type Miles = Int |
type Miles = Int |
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| Line 24: | Line 24: | ||
The functions toMiles and fromMiles is already effectively id ! |
The functions toMiles and fromMiles is already effectively id ! |
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| − | A |
+ | A <hask>newtype</hask> definition just generates a new type whose implementation is the same as the old one. |
| − | same as the old one. |
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| − | 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. |
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| − | So as <hask>undefined</hask> is |
+ | So as <hask>undefined</hask> is Haskell's notation for bottom |
<haskell> |
<haskell> |
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toMiles undefined === undefined |
toMiles undefined === undefined |
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</haskell> |
</haskell> |
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| − | which isn't the case if we use a |
+ | 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 -} |
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</haskell> |
</haskell> |
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| − | For tracking more complex units, you may be able to get some of the way by using [[ |
+ | 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== |
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| − | * For an example toy implementation using [[ |
+ | * For an example toy implementation using [[phantom type]]s & [[functional dependencies]], see [[dimensionalized numbers]] |
* [[Wrapper types]]. |
* [[Wrapper types]]. |
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Latest revision as of 16:35, 13 October 2006
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.
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.
See also
- For an example toy implementation using phantom types & functional dependencies, see dimensionalized numbers
- Wrapper types.