[Haskell-cafe] ANN: prelude-safeenum

[wren ng thornton]

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-- prelude-safeenum 0.1.0
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The prelude-safeenum package offers a safe alternative to the Prelude's
Enum class in order to render it safe. While we're at it, we also
generalize the notion of enumeration to support types which can only be
enumerated in one direction.


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-- Description
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The prelude-safeenum package offers an alternative to the notion of
enumeration provided by the Prelude. For now it is just a package, but the
eventual goal is to be incorporated into haskell prime. Some salient
characteristics of the new type-class hierarchy are:


* Removes partial functions:

    The Haskell Language Report defines pred, succ, fromEnum, and toEnum
to be partial functions when the type is Bounded[1], but this is
unacceptable. The new classes remove this problem by correcting the
type signatures for these functions.


* Generalizes the notion of enumeration:

    Rather than requiring that the type is linearly enumerable, we
distinguish between forward enumeration (which allows for multiple
predecessors) and backward enumeration (which allows for multiple
successors).


* Adds new functions: enumDownFrom, enumDownFromTo

    One of the big problems with the partiality of pred is that there is
no safe way to enumerate downwards since in the border case
(enumFromThen x (pred x)) will throw an error rather than evaluating
to [x] as desired. These new functions remove this problem.


* Removes the requirement that the enumeration order coincides with the
Ord ordering (if one exists). Though, of course, it's advisable to keep
them in sync if possible, for your sanity.


* Ensures that the notion of enumeration is well-defined:

    This much-needed rigor clarifies the meaning of enumeration. In
addition, it rules out instances for Float and Double which are highly
problematic and often confuse newcomers to Haskell. Unfortunately,
this rigor does render the instance for Ratio problematic. However,
Ratio instances /can/ be provided so long as the base type is
enumerable (and Integral, naturally); but they must be done in an
obscure order that does not coincide with Ord.


* The obscure order required for well-defined enumeration of Ratio is
provided.


[1]
<http://www.haskell.org/onlinereport/haskell2010/haskellch6.html#x13-1310006.3.4>


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-- Links
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Homepage:
    http://code.haskell.org/~wren/

Hackage:
    http://hackage.haskell.org/package/prelude-safeenum

Darcs:
    http://community.haskell.org/~wren/prelude-safeenum

Haddock (Darcs version):
    http://community.haskell.org/~wren/prelude-safeenum/dist/doc/html/prelude-safeenum

-- 
Live well,
~wren