data-default-extra: A class for types with a default value.
This is a package candidate release! Here you can preview how this package release will appear once published to the main package index (which can be accomplished via the 'maintain' link below). Please note that once a package has been published to the main package index it cannot be undone! Please consult the package uploading documentation for more information.
An alternative to
data-default package, that
provides a lot of additional instances for Default type class.
Default type class provides a default value (def) of a type. In case when
a particluar type has an instance for Monoid, then the default value should
be mempty, otherwise it can cause a great deal of confusion.
One must understand, that Default type class, has no axioms associated with
it. Its purpose is to minimize:
Number of imports.
Number of introduced definitions that are part of an API.
Complexity of building more complicated "default values" out of simpler "default values".
Cognitive overhead. (Reduction of cognitive overhead is partially a consequence of reducing above issues.)
As one may notice, most of the listed problems, that Default type class
tries to solve, to various degrees of success, are for human benefit, and not
theoretically founded. Because of this, please always try hard to define
sensible instances of Default. Most importantly, document what def means
for particular type, and always check that def is sensible, by testing it
on a real world sample of human beings, one is not a big enough sample.
That said, using Default may not always be a good idea. If it breaks
people's mental model, or theoretical models with real axioms, then just
don't use it.
[Skip to Readme]
Properties
Modules
[Index]
Flags
Automatic Flags
| Name | Description | Default |
|---|---|---|
| with-bytestring | Provide instances for (lazy and strict) ByteString, ByteString Builder and for ShortByteString. | Enabled |
| with-case-insensitive | Provide instance for CI (Case Insensitive) type wrapper. | Enabled |
| with-containers | Provide instances for Set, Map, IntMap, IntSet, Seq, and Tree. | Enabled |
| with-dlist | Provide instance for DList type. | Disabled |
| with-text | Provide instance for (strict and lazy) Text and Text Builder. | Enabled |
| with-old-locale | Provide instance for TimeLocale type. | Enabled |
| with-unordered-containers | Provide instances for HashMap and HashSet. | Enabled |
| with-vector | Provide instances for various (boxed and unboxed) Vector types. | Enabled |
Use -f <flag> to enable a flag, or -f -<flag> to disable that flag. More info
Downloads
- data-default-extra-0.1.0.tar.gz [browse] (Cabal source package)
- Package description (as included in the package)
Maintainer's Corner
Package maintainers
For package maintainers and hackage trustees
Readme for data-default-extra-0.1.0
[back to package description]data-default-extra
Description
An alternative to data-default package, but built on top of the same
underlying package data-default-class. This package provides a lot of
additional instances for Default type class.
Default type class provides a default value (def) of a type. In case when a
particluar type has an instance for Monoid, then the default value should be
mempty, otherwise it can cause a great deal of confusion.
One must understand, that Default type class, has no axioms associated with
it. Its purpose is to minimize:
-
Number of imports.
-
Number of introduced definitions that are part of an API.
-
Complexity of building more complicated "default values" out of simpler "default values".
-
Cognitive overhead. (Reduction of cognitive overhead is partially a consequence of reducing above issues.)
As one may notice, most of the listed problems, that Default type class tries
to solve, to various degrees of success, are for human benefit, and not
theoretically founded. Because of this, please always try hard to define
sensible instances of Default. Most importantly, document what def means
for particular type, and always check that def is sensible, by testing it on
a real world sample of human beings, one is not a big enough sample.
That said, using Default may not always be a good idea. If it breaks people's
mental models, or theoretical models with real axioms, then just don't use it.
This package, in most part, just reexports a set of packages that provide additional instances for data-default-class package.
GHC Generics
Create Default instances using GHC Generics.
Usage example:
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DefaultSignatures #-}
import GHC.Generics (Generic)
import Data.Default (Default)
data MyType = MyType Int (Maybe String)
deriving (Generic, Show)
instance Default MyType
λ> def :: MyType
MyType 0 Nothing
One can also use DeriveAnyClass language extension:
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DeriveAnyClass #-}
import GHC.Generics (Generic)
import Data.Default (Default)
data MyType = MyType Int (Maybe String)
deriving (Default, Generic, Show)
Be aware that DeriveAnyClass doesn't work well together with
GeneralizedNewtypeDeriving on some versions of GHC. See GHC issue #10598
License
The BSD 3-Clause License, see LICENSE file for details.
Contributions
Contributions, pull requests and bug reports are welcome! Please don't be afraid to contact author using GitHub or by e-mail.