| Portability | non-portable |
|---|---|
| Stability | experimental |
| Maintainer | haskelldb-users@lists.sourceforge.net |
Database.HaskellDB.BoundedList
Description
The main idea of bounded lists is to create lists with predetermined maximum size.
BoundedList is a simple, fast and type safe approach to implementing this idea. The implementation is based on inductive instances, making it very easy to expand with new bounds. A new bound only requires one instance of size and two instances of Less.
BoundedList works as follows. Every bound is build up by declaring a data-type representing the new bound. The instance of size only returns the size as an Int. The first instance of Less is for telling the typechecker that this bound is greater than the largest smaller bound. The second instance of Less is used by the typechecker to construct a chain of instances if there is no hardcoded instance available. This way the type checker can determine if a bound is smaller/greater then any other bound.
This inductive approach gives the complexity O(n) on the number of instances and very short type checking times compared to an O(n^2) implementation.
BoundedList also comes with a few utility function for manipulation an contructing bounded lists.
To be noted: Since each bound is a unique type: Explicit shrink and/or grow is needed before using (==). BoundedList does not have an instance of Ordering. (This might change)
- shrink :: (Size n, Size m) => BoundedList a n -> Maybe (BoundedList a m)
- grow :: LessEq n m => BoundedList a n -> BoundedList a m
- trunc :: Size n => [a] -> BoundedList a n
- listBound :: Size n => BoundedList a n -> Int
- toBounded :: Size n => [a] -> Maybe (BoundedList a n)
- fromBounded :: Size n => BoundedList a n -> [a]
- class Size n
- data BoundedList a n
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Documentation
shrink :: (Size n, Size m) => BoundedList a n -> Maybe (BoundedList a m)
Shrinks the BoundedList supplied if
it can do so without truncating the list. Returns Nothing
if the list inside was to long.
grow :: LessEq n m => BoundedList a n -> BoundedList a m
Takes a BoundedList add grows it size.
trunc :: Size n => [a] -> BoundedList a n
Takes a list and transforms it to a BoundedList.
If the list doesn'n fit, the list is truncated
to make it fit into the bounded list.
listBound :: Size n => BoundedList a n -> Int
Returns the length of a BoundedList.
toBounded :: Size n => [a] -> Maybe (BoundedList a n)
Takes a list and transforms it to a BoundedList.
If the list doesn't fit, Nothing is returned.
fromBounded :: Size n => BoundedList a n -> [a]
Takes a BoundedList and return the list inside.
class Size n
Instances
data BoundedList a n
Instances
| (Size n, Eq a) => Eq (BoundedList a n) | |
| (Show a, Size n) => Show (BoundedList a n) |