README.agda

module README where

------------------------------------------------------------------------
-- The Agda standard library, development version
--
-- Author: Nils Anders Danielsson, with contributions from Andreas
-- Abel, Stevan Andjelkovic, Jean-Philippe Bernardy, Peter Berry,
-- Joachim Breitner, Samuel Bronson, Daniel Brown, James Chapman,
-- Liang-Ting Chen, Dominique Devriese, Dan Doel, Érdi Gergő, Helmut
-- Grohne, Simon Foster, Liyang Hu, Patrik Jansson, Alan Jeffrey,
-- Pepijn Kokke, Evgeny Kotelnikov, Eric Mertens, Darin Morrison,
-- Guilhem Moulin, Shin-Cheng Mu, Ulf Norell, Noriyuki OHKAWA, Nicolas
-- Pouillard, Andrés Sicard-Ramírez and Noam Zeilberger.
------------------------------------------------------------------------

-- The development version of the library often requires the latest
-- development version of Agda.

-- Note that no guarantees are currently made about forwards or
-- backwards compatibility, the library is still at an experimental
-- stage.

-- To make use of the library, add the path to the library’s root
-- directory (src) to the Agda search path, either using the
-- --include-path flag or by customising the Emacs mode variable
-- agda2-include-dirs (M-x customize-group RET agda2 RET).

-- To compile the library using the MAlonzo compiler you first need to
-- install some supporting Haskell code, for instance as follows:
--
--   cd ffi
--   cabal install
--
-- Currently the library does not support the Epic or JavaScript
-- compiler backends.

-- Contributions to this library are welcome (but to avoid wasted work
-- it is suggested that you discuss large changes before implementing
-- them). Please send contributions in the form of git pull requests,
-- patch bundles or ask for commmit rights to the repository.  It is
-- appreciated if every patch contains a single, complete change, and
-- if the coding style used in the library is adhered to.

------------------------------------------------------------------------
-- Module hierarchy
------------------------------------------------------------------------

-- The top-level module names of the library are currently allocated
-- as follows:
--
-- • Algebra
--     Abstract algebra (monoids, groups, rings etc.), along with
--     properties needed to specify these structures (associativity,
--     commutativity, etc.), and operations on and proofs about the
--     structures.
-- • Category
--     Category theory-inspired idioms used to structure functional
--     programs (functors and monads, for instance).
-- • Coinduction
--     Support for coinduction.
-- • Data
--     Data types and properties about data types.
-- • Function
--     Combinators and properties related to functions.
-- • Foreign
--     Related to the foreign function interface.
-- • Induction
--     A general framework for induction (includes lexicographic and
--     well-founded induction).
-- • IO
--     Input/output-related functions.
-- • Irrelevance
--     Definitions related to (proscriptive) irrelevance.
-- • Level
--     Universe levels.
-- • Record
--     An encoding of record types with manifest fields and "with".
-- • Reflection
--     Support for reflection.
-- • Relation
--     Properties of and proofs about relations (mostly homogeneous
--     binary relations).
-- • Size
--     Sizes used by the sized types mechanism.
-- • Universe
--     A definition of universes.

------------------------------------------------------------------------
-- A selection of useful library modules
------------------------------------------------------------------------

-- Note that module names in source code are often hyperlinked to the
-- corresponding module. In the Emacs mode you can follow these
-- hyperlinks by typing M-. or clicking with the middle mouse button.

-- • Some data types

import Data.Bool     -- Booleans.
import Data.Char     -- Characters.
import Data.Empty    -- The empty type.
import Data.Fin      -- Finite sets.
import Data.List     -- Lists.
import Data.Maybe    -- The maybe type.
import Data.Nat      -- Natural numbers.
import Data.Product  -- Products.
import Data.Stream   -- Streams.
import Data.String   -- Strings.
import Data.Sum      -- Disjoint sums.
import Data.Unit     -- The unit type.
import Data.Vec      -- Fixed-length vectors.

-- • Some types used to structure computations

import Category.Functor      -- Functors.
import Category.Applicative  -- Applicative functors.
import Category.Monad        -- Monads.

-- • Equality

-- Propositional equality:
import Relation.Binary.PropositionalEquality

-- Convenient syntax for "equational reasoning" using a preorder:
import Relation.Binary.PreorderReasoning

-- Solver for commutative ring or semiring equalities:
import Algebra.RingSolver

-- • Properties of functions, sets and relations

-- Monoids, rings and similar algebraic structures:
import Algebra

-- Negation, decidability, and similar operations on sets:
import Relation.Nullary

-- Properties of homogeneous binary relations:
import Relation.Binary

-- • Induction

-- An abstraction of various forms of recursion/induction:
import Induction

-- Well-founded induction:
import Induction.WellFounded

-- Various forms of induction for natural numbers:
import Induction.Nat

-- • Support for coinduction

import Coinduction

-- • IO

import IO

------------------------------------------------------------------------
-- Record hierarchies
------------------------------------------------------------------------

-- When an abstract hierarchy of some sort (for instance semigroup →
-- monoid → group) is included in the library the basic approach is to
-- specify the properties of every concept in terms of a record
-- containing just properties, parameterised on the underlying
-- operations, sets etc.:
--
--   record IsSemigroup {A} (≈ : Rel A) (∙ : Op₂ A) : Set where
--     open FunctionProperties ≈
--     field
--       isEquivalence : IsEquivalence ≈
--       assoc         : Associative ∙
--       ∙-cong        : ∙ Preserves₂ ≈ ⟶ ≈ ⟶ ≈
--
-- More specific concepts are then specified in terms of the simpler
-- ones:
--
--     record IsMonoid {A} (≈ : Rel A) (∙ : Op₂ A) (ε : A) : Set where
--       open FunctionProperties ≈
--       field
--         isSemigroup : IsSemigroup ≈ ∙
--         identity    : Identity ε ∙
--
--     open IsSemigroup isSemigroup public
--
-- Note here that open IsSemigroup isSemigroup public ensures that the
-- fields of the isSemigroup record can be accessed directly; this
-- technique enables the user of an IsMonoid record to use underlying
-- records without having to manually open an entire record hierarchy.
-- This is not always possible, though. Consider the following definition
-- of preorders:
--
--   record IsPreorder {A : Set}
--                     (_≈_ : Rel A) -- The underlying equality.
--                     (_∼_ : Rel A) -- The relation.
--                     : Set where
--     field
--       isEquivalence : IsEquivalence _≈_
--       -- Reflexivity is expressed in terms of an underlying equality:
--       reflexive     : _≈_ ⇒ _∼_
--       trans         : Transitive _∼_
--
--     module Eq = IsEquivalence isEquivalence
--
--     ...
--
-- The Eq module in IsPreorder is not opened publicly, because it
-- contains some fields which clash with fields or other definitions
-- in IsPreorder.

-- Records packing up properties with the corresponding operations,
-- sets, etc. are sometimes also defined:
--
--   record Semigroup : Set₁ where
--     infixl 7 _∙_
--     infix  4 _≈_
--     field
--       Carrier     : Set
--       _≈_         : Rel Carrier
--       _∙_         : Op₂ Carrier
--       isSemigroup : IsSemigroup _≈_ _∙_
--
--     open IsSemigroup isSemigroup public
--
--     setoid : Setoid
--     setoid = record { isEquivalence = isEquivalence }
--
--   record Monoid : Set₁ where
--     infixl 7 _∙_
--     infix  4 _≈_
--     field
--       Carrier  : Set
--       _≈_      : Rel Carrier
--       _∙_      : Op₂ Carrier
--       ε        : Carrier
--       isMonoid : IsMonoid _≈_ _∙_ ε
--
--     open IsMonoid isMonoid public
--
--     semigroup : Semigroup
--     semigroup = record { isSemigroup = isSemigroup }
--
--     open Semigroup semigroup public using (setoid)
--
-- Note that the Monoid record does not include a Semigroup field.
-- Instead the Monoid /module/ includes a "repackaging function"
-- semigroup which converts a Monoid to a Semigroup.

-- The above setup may seem a bit complicated, but we think it makes the
-- library quite easy to work with, while also providing enough
-- flexibility.

------------------------------------------------------------------------
-- More documentation
------------------------------------------------------------------------

-- Some examples showing where the natural numbers/integers and some
-- related operations and properties are defined, and how they can be
-- used:

import README.Nat
import README.Integer

-- Some examples showing how the AVL tree module can be used.

import README.AVL

-- An example showing how the Record module can be used.

import README.Record

-- An example showing how the case expression can be used.

import README.Case

------------------------------------------------------------------------
-- Core modules
------------------------------------------------------------------------

-- Some modules have names ending in ".Core". These modules are
-- internal, and have (mostly) been created to avoid mutual recursion
-- between modules. They should not be imported directly; their
-- contents are reexported by other modules.

------------------------------------------------------------------------
-- All library modules
------------------------------------------------------------------------

-- For short descriptions of every library module, see Everything:

import Everything

-- Note that the Everything module is generated automatically. If you
-- have downloaded the library from its Git repository and want to
-- type check README then you can (try to) construct Everything by
-- running "cabal install && GenerateEverything".

-- Note that all library sources are located under src or ffi. The
-- modules README, README.* and Everything are not really part of the
-- library, so these modules are located in the top-level directory
-- instead.