# generics-sop

Generic Programming using True Sums of Products

Version on this page: | 0.3.1.0 |

LTS Haskell 9.14: | 0.3.1.0 |

Stackage Nightly 2017-11-21: | 0.3.1.0 |

Latest on Hackage: | 0.3.1.0 |

**andres@well-typed.com**

#### Module documentation for 0.3.1.0

- Generics

A library to support the definition of generic functions. Datatypes are viewed in a uniform, structured way: the choice between constructors is represented using an n-ary sum, and the arguments of each constructor are represented using an n-ary product.

The module `Generics.SOP`

is the main module of this library and contains
more detailed documentation.

Examples of using this library are provided by the following packages:

`basic-sop`

basic examples,`pretty-sop`

generic pretty printing,`lens-sop`

generically computed lenses,`json-sop`

generic JSON conversions.

A detailed description of the ideas behind this library is provided by the paper:

Edsko de Vries and Andres Löh. True Sums of Products. Workshop on Generic Programming (WGP) 2014.

## Changes

# 0.3.1.0 (2017-06-11)

Add AllZip, htrans, hcoerce, hfromI, htoI. These functions are for converting between related structures that do not have common signatures.

The most common application of these functions seems to be the scenario where a datatype has components that are all wrapped in a common type constructor application, e.g. a datatype where every component is a

`Maybe`

. Then we can use`hfromI`

after`from`

to turn the generically derived`SOP`

of`I`

s into an`SOP`

of`Maybe`

s (and back).Add

`IsProductType`

,`IsEnumType`

,`IsWrappedType`

and`IsNewtype`

constraint synonyms capturing specific classes of datypes.

# 0.3.0.0 (2017-04-29)

No longer compatible with GHC 7.6, due to the lack of support for type-level literals.

Support type-level metadata. This is provided by the

`Generics.SOP.Type.Metadata`

module. The two modules`Generics.SOP.Metadata`

and`Generics.SOP.Type.Metadata`

export nearly the same names, so for backwards compatibility, we keep exporting`Generics.SOP.Metadata`

directly from`Generics.SOP`

, whereas`Generics.SOP.Type.Metadata`

is supposed to be imported explicitly (and qualified).Term-level metadata is still available, but is now usually computed automatically from the type-level metadata which contains the same information, using the function

`demoteDatatypeInfo`

. Term-level metadata is unchanged from generics-sop-0.2, so in most cases, even if your code makes use of metadata, you should not need to change anything.If you use TH deriving, then both type-level metadata and term-level metadata is generated for you automatically, for all supported GHC versions.

If you use GGP deriving, then type-level metadata is available if you use GHC 8.0 or newer. If you use GHC 7.x, then GHC.Generics supports only term-level metadata, so we cannot translate that into type-level metadata. In this combination, you cannot use code that relies on type-level metadata, so you should either upgrade GHC or switch to TH-based deriving.

# 0.2.5.0 (2017-04-21)

GHC 8.2 compatibility.

Make

`:.:`

an instance of`Applicative`

,`Foldable`

and`Traversable`

.Add functions

`apInjs'_NP`

and`apInjs'_POP`

. These are variants of`apInjs_NP`

and`apInjs'_POP`

that return their result as an n-ary product, rather than collapsing it into a list.Add

`hexpand`

(and`expand_NS`

and`expand_SOP`

). These functions expand sums into products, given a default value to fill the other slots.Add utility functions such as

`mapII`

or`mapIK`

that lift functions into different combinations of identity and constant functors.Add

`NFData`

(and lifted variants) instances for basic functors, products and sums.

# 0.2.4.0 (2017-02-02)

Add

`hindex`

(and`index_NS`

and`index_SOP`

).Add

`hapInjs`

as a generalization of`apInjs_NP`

and`apInjs_POP`

.Make basic functors instances of lifted classes (such as

`Eq1`

etc).

# 0.2.3.0 (2016-12-04)

Add various metadata getters

Add

`hdicts`

.Add catamorphisms and anamorphisms for

`NP`

and`NS`

.TH compatibility changes for GHC 8.1 (master).

# 0.2.2.0 (2016-07-10)

Introduced

`unZ`

to destruct a unary sum.Add Haddock

`@since`

annotations for various functions.

# 0.2.1.0 (2016-02-08)

Now includes a CHANGELOG.

Should now work with ghc-8.0.1-rc1 and -rc2 (thanks to Oleg Grenrus).

Introduced

`hd`

and`tl`

to project out of a product, and`Projection`

and`projections`

as duals of`Injection`

and`injections`

.

# 0.2.0.0 (2015-10-23)

Now tested with ghc-7.10

Introduced names

`hmap`

,`hcmap`

,`hzipWith`

,`hczipWith`

for`hliftA`

,`hcliftA`

,`hliftA2`

,`hcliftA2`

, respectively. Similarly for the specialized versions of these functions.The constraint transformers

`All`

and`All2`

are now defined as type classes, not type families. As a consequence, the partial applications`All c`

and`All2 c`

are now possible.Because of the redefinition of

`All`

and`All2`

, some special cases are no longer necessary. For example,`cpure_POP`

can now be implemented as a nested application of`pure_NP`

.Because of the redefinition of

`All`

and`All2`

, the functions`hcliftA'`

and variants (with prime!) are now deprecated. One can easily use the normal versions instead. For example, the definition of`hcliftA'`

is now simply`hcliftA' p = hcliftA (allP p) where allP :: proxy c -> Proxy (All c) allP _ = Proxy`

Because

`All`

and`All2`

are now type classes, they now have superclass constraints implying that the type-level lists they are ranging over must have singletons.`class (SListI xs, ...) => All c xs class (SListI xss, ...) => All2 c xss`

Some type signatures can be simplified due to this.

The

`SingI`

typeclass and`Sing`

datatypes are now deprecated. The replacements are called`SListI`

and`SList`

. The`sing`

method is now called`sList`

. The difference is that the new versions reveal only the spine of the list, and contain no singleton representation for the elements anymore.For one-dimensional type-level lists, replace

`SingI xs => ...`

by

`SListI xs => ...`

For two-dimensional type-level lists, replace

`SingI xss => ...`

by

`All SListI xss => ...`

Because All itself implies

`SListI xss`

(see above), this constraint is equivalent to the old`Sing xss`

.The old names are provided for (limited) backward compatibility. They map to the new constructs. This will work in some, but not all scenarios.

The function

`lengthSing`

has also been renamed to`lengthSList`

for consistency, and the old name is deprecated.All

`Proxy c`

arguments have been replaced by`proxy c`

flexible arguments, so that other type constructors can be used as proxies.Class-level composition (

`Compose`

), pairing (`And`

), and a trivial constraint (`Top`

) have been added. Type-level map (`Map`

) has been removed. Occurrences such as`All c (Map f xs)`

should now be replaced with

`All (c `Compose` f) xs`

There is a new module called

`Generics.SOP.Dict`

that contains functions for manipulating dictionaries explicitly. These can be used to prove theorems about non-trivial class constraints such as the ones that get built using`All`

and`All2`

. Some such theorems are provided.There is a new TH function

`deriveGenericFunctions`

that derives the code of a datatype and conversion functions, but does not create a class instance. (Contributed by Oleg Grenrus.)There is a new TH function

`deriveMetadataValue`

that derives a`DatatypeInfo`

value for a datatype, but does not create an instance of`HasDatatypeInfo`

. (Contributed by Oleg Grenrus.)There is a very simple example file. (Contributed by Oleg Grenrus.)

The function

`hcollapse`

for`NS`

now results in an`a`

rather than an`I a`

, matching the specialized version`collapse_NS`

. (Suggested by Roman Cheplyaka.)

# 0.1.1.2 (2015-03-27)

Updated version bounds for ghc-prim (for ghc-7.10).

# 0.1.1.1 (2015-03-20)

Preparations for ghc-7.10.

Documentation fix. (Contributed by Roman Cheplyaka.)

# 0.1.1 (2015-01-06)

Documentation fixes.

Add superclass constraint (TODO).

Now derive tuple instance for tuples up to 30 components. (Contributed by Michael Orlitzky.)