The semigroupoids package

[Tags:bsd3, library]

Provides a wide array of semigroupoids and operations for working with semigroupds.

A Semigroupoid is a Category without the requirement of identity arrows for every object in the category.

When working with comonads you often have the <*> portion of an Applicative, but not the pure. This was captured in Uustalu and Vene's "Essence of Dataflow Programming" in the form of the ComonadZip class in the days before Applicative. Apply provides a weaker invariant, but for the comonads used for data flow programming (found in the streams package), this invariant is preserved. Applicative function composition forms a semigroupoid.

Similarly many structures are nearly a comonad, but not quite, for instance lists provide a reasonable extend operation in the form of tails, but do not always contain a value.

Ideally the following relationships would hold:

 Traversable <---- Foldable <--- Functor ------> Alt ---------> Plus           Semigroupoid
      |               |            |                              |                  |
      v               v            v                              v                  v
 Traversable1 <--- Foldable1     Apply --------> Applicative -> Alternative      Category
                                   |               |              |                  |
                                   v               v              v                  v
                                 Bind ---------> Monad -------> MonadPlus          Arrow

Apply, Bind, and Extend (not shown) give rise the Static, Kleisli and Cokleisli semigroupoids respectively.

This lets us remove many of the restrictions from various monad transformers as in many cases the binding operation or <*> operation does not require them.

Finally, to work with these weaker structures it is beneficial to have containers that can provide stronger guarantees about their contents, so versions of Traversable and Foldable that can be folded with just a Semigroup are added.


Versions 1.0.0, 1.1.0, 1.1.1, 1.1.2, 1.1.3, 1.2.0, 1.2.1, 1.2.2,,,,, 1.2.4, 1.2.5, 1.2.6,,, 1.3, 1.3.1,,, 1.3.2,, 1.3.3, 1.3.4, 3.0,,, 3.0.1, 3.0.2, 3.0.3, 3.1, 4.0, 4.0.1, 4.0.2,, 4.0.3, 4.0.4, 4.2, 4.3, 4.5, 5,,,,, 5.0.1, 5.1, 5.2 (info)
Dependencies base (==4.*), comonad (==3.*), containers (>=0.3 && <0.6), contravariant (>= && <1), semigroups (>= && <1), transformers (>=0.2 && <0.4) [details]
License BSD3
Copyright Copyright (C) 2011 Edward A. Kmett
Author Edward A. Kmett
Maintainer Edward A. Kmett <>
Category Control, Comonads
Home page
Bug tracker
Source repository head: git clone git://
Uploaded Sun Feb 3 10:30:54 UTC 2013 by EdwardKmett
Distributions Arch:5.1, Debian:, FreeBSD:, LTSHaskell:5.1, NixOS:5.2, Stackage:5.1, Tumbleweed:5.1
Downloads 143052 total (6775 in the last 30 days)
4 []
Status Docs uploaded by user
Build status unknown [no reports yet]




Maintainer's Corner

For package maintainers and hackage trustees