The data-default-extra package

[Tags:bsd3, library]

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:

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.


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Properties

Versions 0.0.1, 0.1.0
Change log ChangeLog.md
Dependencies base (<6), data-default-class (>=0.0.1 && <0.2), data-default-instances-bytestring (==0.0.*), data-default-instances-case-insensitive (==0.0.*), data-default-instances-containers (==0.0.*), data-default-instances-dlist (==0.0.*), data-default-instances-new-base (>=0.0.1 && <0.2), data-default-instances-old-locale (==0.0.*), data-default-instances-text (==0.0.*), data-default-instances-unordered-containers (==0.0.*), data-default-instances-vector (==0.0.*), ghc-prim [details]
License BSD3
Copyright (c) 2016-2017 Peter Trško
Author Peter Trško
Maintainer peter.trsko@gmail.com
Category Data
Home page https://github.com/trskop/data-default-extra
Bug tracker https://github.com/trskop/data-default-extra/issues
Source repository head: git clone git://github.com/trskop/data-default-extra.git
this: git clone git://github.com/trskop/data-default-extra.git(tag class-0.1.2)
Uploaded Thu Jan 5 22:23:05 UTC 2017 by PeterTrsko
Distributions NixOS:0.1.0
Downloads 96 total (4 in the last 30 days)
Votes
0 []
Status Docs available [build log]
Last success reported on 2017-01-05 [all 1 reports]

Modules

[Index]

Flags

NameDescriptionDefaultType
with-bytestringProvide instances for (lazy and strict) ByteString, ByteString Builder and for ShortByteString.EnabledAutomatic
with-case-insensitiveProvide instance for CI (Case Insensitive) type wrapper.EnabledAutomatic
with-containersProvide instances for Set, Map, IntMap, IntSet, Seq, and Tree.EnabledAutomatic
with-dlistProvide instance for DList type.DisabledAutomatic
with-textProvide instance for (strict and lazy) Text and Text Builder.EnabledAutomatic
with-old-localeProvide instance for TimeLocale type.EnabledAutomatic
with-unordered-containersProvide instances for HashMap and HashSet.EnabledAutomatic
with-vectorProvide instances for various (boxed and unboxed) Vector types.EnabledAutomatic

Use -f <flag> to enable a flag, or -f -<flag> to disable that flag. More info

Downloads

Maintainer's Corner

For package maintainers and hackage trustees

Readme for data-default-extra

Readme for data-default-extra-0.1.0

data-default-extra

Hackage Hackage Dependencies Haskell Programming Language BSD3 License

Build

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.