added Relations

src/Naturals.lagda

@@ -456,6 +456,9 @@ definition to equivalent inference rules for judgements about equality.
     ------------
     zero + n = n
 
+    m : ℕ
+    n : ℕ
+    p : ℕ
     m + n = p
     -------------------
     (suc m) + n = suc p

src/Properties.lagda

@@ -21,6 +21,7 @@ everything in the previous chapter is also found in the library module
 We also require propositional equality. -->
 
 \begin{code}
+open import Naturals using (ℕ; zero; suc; _+_; _*_; _∸_)
 open import Relation.Binary.PropositionalEquality using (_≡_; refl)
 \end{code}
 
@@ -72,15 +73,9 @@ The answer is yes! We can prove a property holds for all naturals using
 
 ## Proof by induction
 
-Recall the definition of natural numbers.
-\begin{code}
-data ℕ : Set where
-  zero : ℕ
-  suc : ℕ → ℕ
-\end{code}
-This tells us that `zero` is a natural---the *base case*---and that if
-`m` is a natural then `suc m` is also a natural---the *inductive
-case*.
+Recall the definition of natural numbers consists of a *base case*
+which tells us that `zero` is a natural, and an *inductive case*
+which tells us that if `m` is a natural then `suc m` is also a natural.
 
 Proofs by induction follow the structure of this definition.  To prove
 a property of natural numbers by induction, we need prove two cases.
@@ -173,12 +168,10 @@ we must show to hold, become:
 In the inference rules, `n` and `p` are any arbitary natural numbers, so when we
 are done with the proof we know it holds for any `n` and `p` as well as any `m`.
 
-Recall the definition of addition.
-\begin{code}
-_+_ : ℕ → ℕ → ℕ
-zero    + n  =  n                -- (i)
-(suc m) + n  =  suc (m + n)      -- (ii)
-\end{code}
+Recall the definition of addition has two clauses. 
+
+    zero    + n  =  n                -- (i)
+    (suc m) + n  =  suc (m + n)      -- (ii)
 
 For the base case, we must show:
 
@@ -561,10 +554,9 @@ com+suc : ∀ (m n : ℕ) → n + suc m ≡ suc (n + m)
 com+suc m zero = refl
 com+suc m (suc n) rewrite com+suc m n = refl
 
-com : ∀ (m n : ℕ) → m + n ≡ n + m
-com zero n rewrite com+zero n = refl
-com (suc m) n rewrite com+suc m n | com m n = refl
-
+com+ : ∀ (m n : ℕ) → m + n ≡ n + m
+com+ zero n rewrite com+zero n = refl
+com+ (suc m) n rewrite com+suc m n | com+ m n = refl
 \end{code}
 Here we have renamed Lemma (x) and (xi) to `com+zero` and `com+suc`,
 respectively.  In the final line, rewriting with two equations

src/Relations.lagda

@@ -0,0 +1,158 @@
+---
+title     : "Relations: Inductive Definition of Relations"
+layout    : page
+permalink : /Relations
+---
+
+After having defined operations such as addition and multiplication,
+the next step is to define relations, such as *less than or equal*.
+
+## Imports
+
+\begin{code}
+open import Naturals using (ℕ; zero; suc; _+_; _*_; _∸_)
+open import Properties using (com+; com+zero; com+suc)
+open import Relation.Binary.PropositionalEquality using (_≡_; refl)
+\end{code}
+
+## Defining relations
+
+The relation *less than or equal* has an infinite number of
+instances.  Here are just a few of them:
+
+   0 ≤ 0     0 ≤ 1     0 ≤ 2     0 ≤ 3     ...
+             1 ≤ 1     1 ≤ 2     1 ≤ 3     ...
+                       2 ≤ 2     2 ≤ 3     ...
+                                 3 ≤ 3     ...
+                                           ...
+
+And yet, we can write a finite definition that encompasses
+all of these instances in just a few lines.  Here is the
+definition as a pair of inference rules:
+
+    z≤n --------
+        zero ≤ n
+
+        m ≤ n
+    s≤s -------------
+        suc m ≤ suc n
+
+And here is the definition in Agda:
+\begin{code}
+data _≤_ : ℕ → ℕ → Set where
+  z≤n : ∀ {m : ℕ} → zero ≤ m
+  s≤s : ∀ {m n : ℕ} → m ≤ n → suc m ≤ suc n
+\end{code}
+Here `z≤n` and `s≤s` (with no spaces) are constructor names,
+while `m ≤ m`, and `m ≤ n` and `suc m ≤ suc n` (with spaces)
+are propositions.
+
+Both definitions above tell us the same two things:
+
++ *Base case*: for all naturals `n`, the proposition `zero ≤ n` holds
++ *Inductive case*: for all naturals `m` and `n`, if the proposition
+  `m ≤ n` holds, then the proposition `suc m ≤ suc n` holds.
+
+In fact, they each give us a bit more detail:
+
++ *Base case*: for all naturals `n`, the constructor `z≤n` 
+  produces evidence that `zero ≤ n` holds.
++ *Inductive case*: for all naturals `m` and `n`, the constructor
+  `s≤s` takes evidence that `m ≤ n` holds into evidence that
+  `suc m ≤ suc n` holds.
+
+Here we have used the word *evidence* as interchangeable with the
+word *proof*.  We will tend to say *evidence* when we want to stress
+that proofs are just terms in Agda.
+
+For example, here in inference rule notation is the proof that
+`2 ≤ 4`.
+
+      z≤n -----
+          0 ≤ 2
+     s≤s -------
+          1 ≤ 3
+    s≤s ---------
+          2 ≤ 4
+
+And here is the corresponding Agda proof.
+\begin{code}
+ex₁ : 2 ≤ 4
+ex₁ = s≤s (s≤s z≤n)
+\end{code}
+
+## Implicit arguments
+
+In the Agda definition, the two lines defining the constructors
+use `∀`, very similar to our use of `∀` in propositions such as:
+
+    com+ : ∀ (m n : ℕ) → m + n ≡ n + m
+
+However, here the declarations are surrounded by curly braces `{ }`
+rather than parentheses `( )`.  This means that the arguments are
+*implicit* and need not be written explicitly.  Thus, we would write,
+for instance, `com+ m n` for the proof that `m + n ≡ n + m`, but
+here will write `z≤n` for the proof that `m ≤ m`, leaving the `m` implicit,
+or if `m≤n` is evidence that `m ≤ n`, we write write `s≤s m≤n` for the
+evidence that `suc m ≤ suc n`, leaving both `m` and `n` implicit.
+
+It is possible to provide implicit arguments explicitly if we wish.
+For instance, here is the Agda proof that `2 ≤ 4` repeated, with the
+implicit arguments made explicit.
+\begin{code}
+ex₂ : 2 ≤ 4
+ex₂ = s≤s {1} {3} (s≤s {0} {2} (z≤n {2}))
+\end{code}
+
+## Proving properties of inductive definitions.
+
+\begin{code}
+infix 4 _≤_
+
+refl≤ : ∀ (n : ℕ) → n ≤ n
+refl≤ zero = z≤n
+refl≤ (suc n) = s≤s (refl≤ n)
+
+trans≤ : ∀ {m n p : ℕ} → m ≤ n → n ≤ p → m ≤ p
+trans≤ z≤n n≤p = z≤n
+trans≤ (s≤s m≤n) (s≤s n≤p) = s≤s (trans≤ m≤n n≤p)
+
+antisym≤ : ∀ {m n : ℕ} → m ≤ n → n ≤ m → m ≡ n
+antisym≤ z≤n z≤n = refl
+antisym≤ (s≤s m≤n) (s≤s n≤m) rewrite antisym≤ m≤n n≤m = refl
+
+mono+≤ : ∀ (m p q : ℕ) → p ≤ q → m + p ≤ m + q
+mono+≤ zero p q p≤q =  p≤q
+mono+≤ (suc m) p q p≤q =  s≤s (mono+≤ m p q p≤q)
+
+mono+≤′ : ∀ (m n p : ℕ) → m ≤ n → m + p ≤ n + p
+mono+≤′ m n p m≤n rewrite com+ m p | com+ n p = mono+≤ p m n m≤n
+
+mono+≤″ : ∀ (m n p q : ℕ) → m ≤ n → p ≤ q → m + p ≤ n + q
+mono+≤″ m n p q m≤n p≤q = trans≤ (mono+≤′ m n p m≤n) (mono+≤ n p q p≤q)
+
+inverse+∸≤ : ∀ (m n : ℕ) → m ≤ n → (n ∸ m) + m ≡ n
+inverse+∸≤ zero n z≤n rewrite com+zero n  = refl
+inverse+∸≤ (suc m) (suc n) (s≤s m≤n)
+  rewrite com+suc m (n ∸ m) | inverse+∸≤ m n m≤n = refl
+
+data _<_ : ℕ → ℕ → Set where
+  z<s : ∀ {n : ℕ} → zero < suc n
+  s<s : ∀ {m n : ℕ} → m < n → suc m < suc n
+
+infix 4 _<_
+
+<implies≤ : ∀ (m n : ℕ) → m < n → suc m ≤ n
+<implies≤ m n m<n = ?
+
+≤implies< : ∀ (m n : ℕ) → suc m ≤ n → m < n
+≤implies< m n m≤n = ?
+\end{code}
+
+## Unicode
+
+In this chapter we use the following unicode.
+
+    ≡  U+2261  IDENTICAL TO (\==)
+    ∀  U+2200  FOR ALL (\forall)
+    λ  U+03BB  GREEK SMALL LETTER LAMBDA (\Gl, \lambda)