# generics-sop

Generic Programming using True Sums of Products

Version on this page: | 0.2.4.0 |

LTS Haskell 8.2: | 0.2.4.0 |

Stackage Nightly 2017-02-20: | 0.2.4.0 |

Latest on Hackage: | 0.2.4.0 |

**andres@well-typed.com**

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

# 0.2.4.0 (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.)