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<h2>Agda Full Example</h2>
<p>** Copyright - this piece of code is a modified version of [https://plfa.github.io/Naturals/] by Philip Wadler, Wen Kokke, Jeremy G Siek and contributors.</p>
<pre><code>-- 1.1 Naturals

module plfa.part1.Naturals where

-- The standard way to enter Unicode math symbols in Agda
-- is to use the IME provided by agda-mode.
-- for example ℕ can be entered by typing \bN.

-- The inductive definition of natural numbers.
-- In Agda, data declarations correspond to axioms.
-- Also types correspond to sets.
-- CHAR: \bN → ℕ
data ℕ : Set where
	-- This corresponds to the `zero` rule in Dedekin-Peano's axioms.
	-- Note that the syntax resembles Haskell GADTs.
	-- Also note that the `has-type` operator is `:` (as in Idris), not `::` (as in Haskell).
	zero : ℕ
	-- This corresponds to the `succesor` rule in Dedekin-Peano's axioms.
	-- In such a constructive system in Agda, induction rules etc comes by nature.
	-- The function arrow can be either `->` or `→`.
	-- CHAR: \to or \-> or \r- → →
	suc  : ℕ → ℕ

-- EXERCISE `seven`
seven : ℕ
seven = suc (suc (suc (suc (suc (suc (suc zero))))))

-- This line is a compiler pragma.
-- It makes `ℕ` correspond to Haskell's type `Integer`
-- and allows us to use number literals (0, 1, 2, ...) to express `ℕ`.
{-# BUILTIN NATURAL ℕ #-}

-- Agda has a module system corresponding to the project file structure.
-- e.g. `My.Namespace` is in
-- `project path/My/Namespace.agda`.

-- The `import` statement does NOT expose the names to the top namespace.
-- You'll have to use `My.Namespace.thing` instead of directly `thing`.
import Relation.Binary.PropositionalEquality as Eq
-- The `open` statement unpacks all the names in a imported namespace and exposes them to the top namespace.
-- Alternatively the `open import` statement imports a namespace and opens it at the same time.
-- The `using (a; ..)` clause can limit a range of names to expose, instead of all of them.
-- Alternatively, the `hiding (a; ..)` clause can limit a range of names NOT to expose.
-- Also the `renaming (a to b; ..)` clause can rename names.
-- CHAR: \== → ≡
--       \gt → ⟨
--       \lt → ⟩
--       \qed → ∎
open Eq using (_≡_; refl)
open Eq.≡-Reasoning using (begin_; _≡⟨⟩_; _∎)

-- Addition of `ℕ`.
-- Note that Agda functions are *like* Haskell functions.
-- In Agda, operators can be mixfix (incl. infix, prefix, suffix, self-bracketing and many others).
-- All the `holes` are represented by `_`s. Unlike Haskell, operator names don't need to be put in parentheses.
-- Operators can also be called in the manner of normal functions.
-- e.g. a + b = _+_ a b.
-- Sections are also available, though somehow weird.
-- e.g. a +_ = _+_ a.
_+_ : ℕ → ℕ → ℕ
-- Lhs can also be infix!
-- This is the standard definition in both Peano and Agda stdlib.
-- We do pattern match on the first parameter, it's both convention and for convenience.
-- Agda does a termination check on recursive function.
-- Here the first parameter decreases over evaluation thus it is *well-founded*.
zero    + n = n
(suc m) + n = suc (m + n)

-- Here we take a glance at the *dependent type*.
-- In dependent type, we can put values into type level, and vice versa.
-- This is especially useful when we're expecting to make the types more precise.
-- Here `_≡_` is a type that says that two values are *the same*, that is, samely constructed.
_ : 2 + 3 ≡ 5
-- We can do it by ≡-Reasoning, that is writing a (unreasonably long) chain of equations.
_ =
	begin
		2 + 3
	≡⟨⟩ -- This operator means the lhs and rhs can be reduced to the same form so that they are equal.
		suc (1 + 3)
	≡⟨⟩
		suc (suc (0 + 3)) -- Just simulating the function evaluation
	≡⟨⟩
		suc (suc 3)
	≡⟨⟩
		5
	∎ -- The *tombstone*, QED.

-- Well actually we can also get this done by simply writing `refl`.
-- `refl` is a proof that says "If two values evaluates to the same form, then they are equal".
-- Since Agda can automatically evaluate the terms for us, this makes sense.
_ : 2 + 3 ≡ 5
_ = refl

-- Multiplication of `ℕ`, defined with addition.
_*_ : ℕ → ℕ → ℕ
-- Here we can notice that in Agda we prefer to indent by *visual blocks* instead by a fixed number of spaces.
zero    * n = zero
-- Here the addition is at the front, to be consistent with addition.
(suc m) * n = n + (m * n)

-- EXERCISE `_^_`, Exponentation of `ℕ`.
_^_ : ℕ → ℕ → ℕ
-- We can only pattern match the 2nd argument.
m ^ zero    = suc zero
m ^ (suc n) = m * (m ^ n)

-- *Monus* (a wordplay on minus), the non-negative subtraction of `ℕ`.
-- if less than 0 then we get 0.
-- CHAR: \.- → ∸
_∸_ : ℕ → ℕ → ℕ
m     ∸ zero  = m
zero  ∸ suc n = zero
suc m ∸ suc n = m ∸ n

-- Now we define the precedence of the operators, as in Haskell.
infixl 6 _+_ _∸_
infixl 7 _*_

-- These are some more pragmas. Should be self-explaining.
{-# BUILTIN NATPLUS _+_ #-}
{-# BUILTIN NATTIMES _*_ #-}
{-# BUILTIN NATMINUS _∸_ #-}

-- EXERCISE `Bin`. We define a binary representation of natural numbers.
-- Leading `O`s are acceptable.
data Bin : Set where
	⟨⟩ : Bin
	_O : Bin → Bin
	_I : Bin → Bin

-- Like `suc` for `Bin`.
inc : Bin → Bin
inc ⟨⟩ = ⟨⟩ I
inc (b O) = b I
inc (b I) = (inc b) O

-- `ℕ` to `Bin`. This is a Θ(n) solution and awaits a better Θ(log n) reimpelementation.
to : ℕ → Bin
to zero    = ⟨⟩ O
to (suc n) = inc (to n)

-- `Bin` to `ℕ`.
from : Bin → ℕ
from ⟨⟩    = 0
from (b O) = 2 * (from b)
from (b I) = 1 + 2 * (from b)

-- Simple tests from 0 to 4.
_ : from (to 0) ≡ 0
_ = refl

_ : from (to 1) ≡ 1
_ = refl

_ : from (to 2) ≡ 2
_ = refl

_ : from (to 3) ≡ 3
_ = refl

_ : from (to 4) ≡ 4
_ = refl

-- EXERCISE END `Bin`

-- STDLIB: import Data.Nat using (ℕ; zero; suc; _+_; _*_; _^_; _∸_)
</code></pre>
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