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Basic.lean
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Basic.lean
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/-
Copyright (c) 2016 Microsoft Corporation. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Author: Leonardo de Moura
-/
prelude
import Init.SimpLemmas
import Init.Data.Nat.Basic
set_option linter.missingDocs true -- keep it documented
open Decidable List
universe u v w
variable {α : Type u} {β : Type v} {γ : Type w}
namespace List
instance : GetElem (List α) Nat α fun as i => i < as.length where
getElem as i h := as.get ⟨i, h⟩
@[simp] theorem cons_getElem_zero (a : α) (as : List α) (h : 0 < (a :: as).length) : getElem (a :: as) 0 h = a := by
rfl
@[simp] theorem cons_getElem_succ (a : α) (as : List α) (i : Nat) (h : i + 1 < (a :: as).length) : getElem (a :: as) (i+1) h = getElem as i (Nat.lt_of_succ_lt_succ h) := by
rfl
theorem length_add_eq_lengthTRAux (as : List α) (n : Nat) : as.length + n = as.lengthTRAux n := by
induction as generalizing n with
| nil => simp [length, lengthTRAux]
| cons a as ih =>
simp [length, lengthTRAux, ← ih, Nat.succ_add]
rfl
@[csimp] theorem length_eq_lengthTR : @List.length = @List.lengthTR := by
apply funext; intro α; apply funext; intro as
simp [lengthTR, ← length_add_eq_lengthTRAux]
@[simp] theorem length_nil : length ([] : List α) = 0 :=
rfl
/-- Auxiliary for `List.reverse`. `List.reverseAux l r = l.reverse ++ r`, but it is defined directly. -/
def reverseAux : List α → List α → List α
| [], r => r
| a::l, r => reverseAux l (a::r)
/--
`O(|as|)`. Reverse of a list:
* `[1, 2, 3, 4].reverse = [4, 3, 2, 1]`
Note that because of the "functional but in place" optimization implemented by Lean's compiler,
this function works without any allocations provided that the input list is unshared:
it simply walks the linked list and reverses all the node pointers.
-/
def reverse (as : List α) : List α :=
reverseAux as []
theorem reverseAux_reverseAux_nil (as bs : List α) : reverseAux (reverseAux as bs) [] = reverseAux bs as := by
induction as generalizing bs with
| nil => rfl
| cons a as ih => simp [reverseAux, ih]
theorem reverseAux_reverseAux (as bs cs : List α) : reverseAux (reverseAux as bs) cs = reverseAux bs (reverseAux (reverseAux as []) cs) := by
induction as generalizing bs cs with
| nil => rfl
| cons a as ih => simp [reverseAux, ih (a::bs), ih [a]]
@[simp] theorem reverse_reverse (as : List α) : as.reverse.reverse = as := by
simp [reverse]; rw [reverseAux_reverseAux_nil]; rfl
/--
`O(|xs|)`: append two lists. `[1, 2, 3] ++ [4, 5] = [1, 2, 3, 4, 5]`.
It takes time proportional to the first list.
-/
protected def append : (xs ys : List α) → List α
| [], bs => bs
| a::as, bs => a :: List.append as bs
/-- Tail-recursive version of `List.append`. -/
def appendTR (as bs : List α) : List α :=
reverseAux as.reverse bs
@[csimp] theorem append_eq_appendTR : @List.append = @appendTR := by
apply funext; intro α; apply funext; intro as; apply funext; intro bs
simp [appendTR, reverse]
induction as with
| nil => rfl
| cons a as ih =>
simp [reverseAux, List.append, ih, reverseAux_reverseAux]
instance : Append (List α) := ⟨List.append⟩
@[simp] theorem nil_append (as : List α) : [] ++ as = as := rfl
@[simp] theorem append_nil (as : List α) : as ++ [] = as := by
induction as with
| nil => rfl
| cons a as ih =>
simp_all [HAppend.hAppend, Append.append, List.append]
@[simp] theorem cons_append (a : α) (as bs : List α) : (a::as) ++ bs = a::(as ++ bs) := rfl
@[simp] theorem append_eq (as bs : List α) : List.append as bs = as ++ bs := rfl
theorem append_assoc (as bs cs : List α) : (as ++ bs) ++ cs = as ++ (bs ++ cs) := by
induction as with
| nil => rfl
| cons a as ih => simp [ih]
theorem append_cons (as : List α) (b : α) (bs : List α) : as ++ b :: bs = as ++ [b] ++ bs := by
induction as with
| nil => simp
| cons a as ih => simp [ih]
instance : EmptyCollection (List α) := ⟨List.nil⟩
/--
`O(|l|)`. `erase l a` removes the first occurrence of `a` from `l`.
* `erase [1, 5, 3, 2, 5] 5 = [1, 3, 2, 5]`
* `erase [1, 5, 3, 2, 5] 6 = [1, 5, 3, 2, 5]`
-/
protected def erase {α} [BEq α] : List α → α → List α
| [], _ => []
| a::as, b => match a == b with
| true => as
| false => a :: List.erase as b
/--
`O(i)`. `eraseIdx l i` removes the `i`'th element of the list `l`.
* `erase [a, b, c, d, e] 0 = [b, c, d, e]`
* `erase [a, b, c, d, e] 1 = [a, c, d, e]`
* `erase [a, b, c, d, e] 5 = [a, b, c, d, e]`
-/
def eraseIdx : List α → Nat → List α
| [], _ => []
| _::as, 0 => as
| a::as, n+1 => a :: eraseIdx as n
/--
`O(1)`. `isEmpty l` is true if the list is empty.
* `isEmpty [] = true`
* `isEmpty [a] = false`
* `isEmpty [a, b] = false`
-/
def isEmpty : List α → Bool
| [] => true
| _ :: _ => false
/--
`O(|l|)`. `map f l` applies `f` to each element of the list.
* `map f [a, b, c] = [f a, f b, f c]`
-/
@[specialize] def map (f : α → β) : List α → List β
| [] => []
| a::as => f a :: map f as
/-- Tail-recursive version of `List.map`. -/
@[inline] def mapTR (f : α → β) (as : List α) : List β :=
loop as []
where
@[specialize] loop : List α → List β → List β
| [], bs => bs.reverse
| a::as, bs => loop as (f a :: bs)
theorem reverseAux_eq_append (as bs : List α) : reverseAux as bs = reverseAux as [] ++ bs := by
induction as generalizing bs with
| nil => simp [reverseAux]
| cons a as ih =>
simp [reverseAux]
rw [ih (a :: bs), ih [a], append_assoc]
rfl
@[simp] theorem reverse_nil : reverse ([] : List α) = [] :=
rfl
@[simp] theorem reverse_cons (a : α) (as : List α) : reverse (a :: as) = reverse as ++ [a] := by
simp [reverse, reverseAux]
rw [← reverseAux_eq_append]
@[simp] theorem reverse_append (as bs : List α) : (as ++ bs).reverse = bs.reverse ++ as.reverse := by
induction as generalizing bs with
| nil => simp
| cons a as ih => simp [ih]; rw [append_assoc]
theorem mapTR_loop_eq (f : α → β) (as : List α) (bs : List β) :
mapTR.loop f as bs = bs.reverse ++ map f as := by
induction as generalizing bs with
| nil => simp [mapTR.loop, map]
| cons a as ih =>
simp [mapTR.loop, map]
rw [ih (f a :: bs), reverse_cons, append_assoc]
rfl
@[csimp] theorem map_eq_mapTR : @map = @mapTR :=
funext fun α => funext fun β => funext fun f => funext fun as => by
simp [mapTR, mapTR_loop_eq]
/--
`O(|join L|)`. `join L` concatenates all the lists in `L` into one list.
* `join [[a], [], [b, c], [d, e, f]] = [a, b, c, d, e, f]`
-/
def join : List (List α) → List α
| [] => []
| a :: as => a ++ join as
/--
`O(|l|)`. `filterMap f l` takes a function `f : α → Option β` and applies it to each element of `l`;
the resulting non-`none` values are collected to form the output list.
```
filterMap
(fun x => if x > 2 then some (2 * x) else none)
[1, 2, 5, 2, 7, 7]
= [10, 14, 14]
```
-/
@[specialize] def filterMap (f : α → Option β) : List α → List β
| [] => []
| a::as =>
match f a with
| none => filterMap f as
| some b => b :: filterMap f as
/--
`O(|l|)`. `filter f l` returns the list of elements in `l` for which `f` returns true.
```
filter (· > 2) [1, 2, 5, 2, 7, 7] = [5, 7, 7]
```
-/
def filter (p : α → Bool) : List α → List α
| [] => []
| a::as => match p a with
| true => a :: filter p as
| false => filter p as
/-- Tail-recursive version of `List.filter`. -/
@[inline] def filterTR (p : α → Bool) (as : List α) : List α :=
loop as []
where
@[specialize] loop : List α → List α → List α
| [], rs => rs.reverse
| a::as, rs => match p a with
| true => loop as (a::rs)
| false => loop as rs
theorem filterTR_loop_eq (p : α → Bool) (as bs : List α) :
filterTR.loop p as bs = bs.reverse ++ filter p as := by
induction as generalizing bs with
| nil => simp [filterTR.loop, filter]
| cons a as ih =>
simp [filterTR.loop, filter]
split
next => rw [ih, reverse_cons, append_assoc]; simp
next => rw [ih]
@[csimp] theorem filter_eq_filterTR : @filter = @filterTR := by
apply funext; intro α; apply funext; intro p; apply funext; intro as
simp [filterTR, filterTR_loop_eq]
/--
`O(|l|)`. `partition p l` calls `p` on each element of `l`, partitioning the list into two lists
`(l_true, l_false)` where `l_true` has the elements where `p` was true
and `l_false` has the elements where `p` is false.
`partition p l = (filter p l, filter (not ∘ p) l)`, but it is slightly more efficient
since it only has to do one pass over the list.
```
partition (· > 2) [1, 2, 5, 2, 7, 7] = ([5, 7, 7], [1, 2, 2])
```
-/
@[inline] def partition (p : α → Bool) (as : List α) : List α × List α :=
loop as ([], [])
where
@[specialize] loop : List α → List α × List α → List α × List α
| [], (bs, cs) => (bs.reverse, cs.reverse)
| a::as, (bs, cs) =>
match p a with
| true => loop as (a::bs, cs)
| false => loop as (bs, a::cs)
/--
`O(|l|)`. `dropWhile p l` removes elements from the list until it finds the first element
for which `p` returns false; this element and everything after it is returned.
```
dropWhile (· < 4) [1, 3, 2, 4, 2, 7, 4] = [4, 2, 7, 4]
```
-/
def dropWhile (p : α → Bool) : List α → List α
| [] => []
| a::l => match p a with
| true => dropWhile p l
| false => a::l
/--
`O(|l|)`. `find? p l` returns the first element for which `p` returns true,
or `none` if no such element is found.
* `find? (· < 5) [7, 6, 5, 8, 1, 2, 6] = some 1`
* `find? (· < 1) [7, 6, 5, 8, 1, 2, 6] = none`
-/
def find? (p : α → Bool) : List α → Option α
| [] => none
| a::as => match p a with
| true => some a
| false => find? p as
/--
`O(|l|)`. `findSome? f l` applies `f` to each element of `l`, and returns the first non-`none` result.
* `findSome? (fun x => if x < 5 then some (10 * x) else none) [7, 6, 5, 8, 1, 2, 6] = some 10`
-/
def findSome? (f : α → Option β) : List α → Option β
| [] => none
| a::as => match f a with
| some b => some b
| none => findSome? f as
/--
`O(|l|)`. `replace l a b` replaces the first element in the list equal to `a` with `b`.
* `replace [1, 4, 2, 3, 3, 7] 3 6 = [1, 4, 2, 6, 3, 7]`
* `replace [1, 4, 2, 3, 3, 7] 5 6 = [1, 4, 2, 3, 3, 7]`
-/
def replace [BEq α] : List α → α → α → List α
| [], _, _ => []
| a::as, b, c => match a == b with
| true => c::as
| false => a :: replace as b c
/--
`O(|l|)`. `elem a l` or `l.contains a` is true if there is an element in `l` equal to `a`.
* `elem 3 [1, 4, 2, 3, 3, 7] = true`
* `elem 5 [1, 4, 2, 3, 3, 7] = false`
-/
def elem [BEq α] (a : α) : List α → Bool
| [] => false
| b::bs => match a == b with
| true => true
| false => elem a bs
/-- `notElem a l` is `!(elem a l)`. -/
def notElem [BEq α] (a : α) (as : List α) : Bool :=
!(as.elem a)
@[inherit_doc elem] abbrev contains [BEq α] (as : List α) (a : α) : Bool :=
elem a as
/--
`a ∈ l` is a predicate which asserts that `a` is in the list `l`.
Unlike `elem`, this uses `=` instead of `==` and is suited for mathematical reasoning.
* `a ∈ [x, y, z] ↔ a = x ∨ a = y ∨ a = z`
-/
inductive Mem (a : α) : List α → Prop
/-- The head of a list is a member: `a ∈ a :: as`. -/
| head (as : List α) : Mem a (a::as)
/-- A member of the tail of a list is a member of the list: `a ∈ l → a ∈ b :: l`. -/
| tail (b : α) {as : List α} : Mem a as → Mem a (b::as)
instance : Membership α (List α) where
mem := Mem
theorem mem_of_elem_eq_true [DecidableEq α] {a : α} {as : List α} : elem a as = true → a ∈ as := by
match as with
| [] => simp [elem]
| a'::as =>
simp [elem]
split
next h => intros; simp [BEq.beq] at h; subst h; apply Mem.head
next _ => intro h; exact Mem.tail _ (mem_of_elem_eq_true h)
theorem elem_eq_true_of_mem [DecidableEq α] {a : α} {as : List α} (h : a ∈ as) : elem a as = true := by
induction h with
| head _ => simp [elem]
| tail _ _ ih => simp [elem]; split; rfl; assumption
instance [DecidableEq α] (a : α) (as : List α) : Decidable (a ∈ as) :=
decidable_of_decidable_of_iff (Iff.intro mem_of_elem_eq_true elem_eq_true_of_mem)
theorem mem_append_of_mem_left {a : α} {as : List α} (bs : List α) : a ∈ as → a ∈ as ++ bs := by
intro h
induction h with
| head => apply Mem.head
| tail => apply Mem.tail; assumption
theorem mem_append_of_mem_right {b : α} {bs : List α} (as : List α) : b ∈ bs → b ∈ as ++ bs := by
intro h
induction as with
| nil => simp [h]
| cons => apply Mem.tail; assumption
/-- `O(|l|^2)`. Erase duplicated elements in the list.
Keeps the first occurrence of duplicated elements.
* `eraseDups [1, 3, 2, 2, 3, 5] = [1, 3, 2, 5]`
-/
def eraseDups {α} [BEq α] (as : List α) : List α :=
loop as []
where
loop : List α → List α → List α
| [], bs => bs.reverse
| a::as, bs => match bs.elem a with
| true => loop as bs
| false => loop as (a::bs)
/--
`O(|l|)`. Erase repeated adjacent elements. Keeps the first occurrence of each run.
* `eraseReps [1, 3, 2, 2, 2, 3, 5] = [1, 3, 2, 3, 5]`
-/
def eraseReps {α} [BEq α] : List α → List α
| [] => []
| a::as => loop a as []
where
loop {α} [BEq α] : α → List α → List α → List α
| a, [], rs => (a::rs).reverse
| a, a'::as, rs => match a == a' with
| true => loop a as rs
| false => loop a' as (a::rs)
/--
`O(|l|)`. `span p l` splits the list `l` into two parts, where the first part
contains the longest initial segment for which `p` returns true
and the second part is everything else.
* `span (· > 5) [6, 8, 9, 5, 2, 9] = ([6, 8, 9], [5, 2, 9])`
* `span (· > 10) [6, 8, 9, 5, 2, 9] = ([6, 8, 9, 5, 2, 9], [])`
-/
@[inline] def span (p : α → Bool) (as : List α) : List α × List α :=
loop as []
where
@[specialize] loop : List α → List α → List α × List α
| [], rs => (rs.reverse, [])
| a::as, rs => match p a with
| true => loop as (a::rs)
| false => (rs.reverse, a::as)
/--
`O(|l|)`. `groupBy R l` splits `l` into chains of elements
such that adjacent elements are related by `R`.
* `groupBy (·==·) [1, 1, 2, 2, 2, 3, 2] = [[1, 1], [2, 2, 2], [3], [2]]`
* `groupBy (·<·) [1, 2, 5, 4, 5, 1, 4] = [[1, 2, 5], [4, 5], [1, 4]]`
-/
@[specialize] def groupBy (R : α → α → Bool) : List α → List (List α)
| [] => []
| a::as => loop as a [] []
where
@[specialize] loop : List α → α → List α → List (List α) → List (List α)
| a::as, ag, g, gs => match R ag a with
| true => loop as a (ag::g) gs
| false => loop as a [] ((ag::g).reverse::gs)
| [], ag, g, gs => ((ag::g).reverse::gs).reverse
/--
`O(|l|)`. `lookup a l` treats `l : List (α × β)` like an association list,
and returns the first `β` value corresponding to an `α` value in the list equal to `a`.
* `lookup 3 [(1, 2), (3, 4), (3, 5)] = some 4`
* `lookup 2 [(1, 2), (3, 4), (3, 5)] = none`
-/
def lookup [BEq α] : α → List (α × β) → Option β
| _, [] => none
| a, (k,b)::es => match a == k with
| true => some b
| false => lookup a es
/-- `O(|xs|)`. Computes the "set difference" of lists,
by filtering out all elements of `xs` which are also in `ys`.
* `removeAll [1, 1, 5, 1, 2, 4, 5] [1, 2, 2] = [5, 4, 5]`
-/
def removeAll [BEq α] (xs ys : List α) : List α :=
xs.filter (fun x => ys.notElem x)
/--
`O(min n |xs|)`. Removes the first `n` elements of `xs`.
* `drop 0 [a, b, c, d, e] = [a, b, c, d, e]`
* `drop 3 [a, b, c, d, e] = [d, e]`
* `drop 6 [a, b, c, d, e] = []`
-/
def drop : Nat → List α → List α
| 0, a => a
| _+1, [] => []
| n+1, _::as => drop n as
@[simp] theorem drop_nil : ([] : List α).drop i = [] := by
cases i <;> rfl
theorem get_drop_eq_drop (as : List α) (i : Nat) (h : i < as.length) : as[i] :: as.drop (i+1) = as.drop i :=
match as, i with
| _::_, 0 => rfl
| _::_, i+1 => get_drop_eq_drop _ i _
/--
`O(min n |xs|)`. Returns the first `n` elements of `xs`, or the whole list if `n` is too large.
* `take 0 [a, b, c, d, e] = []`
* `take 3 [a, b, c, d, e] = [a, b, c]`
* `take 6 [a, b, c, d, e] = [a, b, c, d, e]`
-/
def take : Nat → List α → List α
| 0, _ => []
| _+1, [] => []
| n+1, a::as => a :: take n as
/--
`O(|xs|)`. Returns the longest initial segment of `xs` for which `p` returns true.
* `takeWhile (· > 5) [7, 6, 4, 8] = [7, 6]`
* `takeWhile (· > 5) [7, 6, 6, 8] = [7, 6, 6, 8]`
-/
def takeWhile (p : α → Bool) : (xs : List α) → List α
| [] => []
| hd :: tl => match p hd with
| true => hd :: takeWhile p tl
| false => []
/--
`O(|l|)`. Applies function `f` to all of the elements of the list, from right to left.
* `foldr f init [a, b, c] = f a <| f b <| f c <| init`
-/
@[specialize] def foldr (f : α → β → β) (init : β) : List α → β
| [] => init
| a :: l => f a (foldr f init l)
/--
`O(|l|)`. Returns true if `p` is true for any element of `l`.
* `any p [a, b, c] = p a || p b || p c`
-/
@[inline] def any (l : List α) (p : α → Bool) : Bool :=
foldr (fun a r => p a || r) false l
/--
`O(|l|)`. Returns true if `p` is true for every element of `l`.
* `all p [a, b, c] = p a && p b && p c`
-/
@[inline] def all (l : List α) (p : α → Bool) : Bool :=
foldr (fun a r => p a && r) true l
/--
`O(|l|)`. Returns true if `true` is an element of the list of booleans `l`.
* `or [a, b, c] = a || b || c`
-/
def or (bs : List Bool) : Bool := bs.any id
/--
`O(|l|)`. Returns true if every element of `l` is the value `true`.
* `and [a, b, c] = a && b && c`
-/
def and (bs : List Bool) : Bool := bs.all id
/--
`O(min |xs| |ys|)`. Applies `f` to the two lists in parallel, stopping at the shorter list.
* `zipWith f [x₁, x₂, x₃] [y₁, y₂, y₃, y₄] = [f x₁ y₁, f x₂ y₂, f x₃ y₃]`
-/
@[specialize] def zipWith (f : α → β → γ) : (xs : List α) → (ys : List β) → List γ
| x::xs, y::ys => f x y :: zipWith f xs ys
| _, _ => []
/--
`O(min |xs| |ys|)`. Combines the two lists into a list of pairs, with one element from each list.
The longer list is truncated to match the shorter list.
* `zip [x₁, x₂, x₃] [y₁, y₂, y₃, y₄] = [(x₁, y₁), (x₂, y₂), (x₃, y₃)]`
-/
def zip : List α → List β → List (Prod α β) :=
zipWith Prod.mk
/--
`O(|l|)`. Separates a list of pairs into two lists containing the first components and second components.
* `unzip [(x₁, y₁), (x₂, y₂), (x₃, y₃)] = ([x₁, x₂, x₃], [y₁, y₂, y₃])`
-/
def unzip : List (α × β) → List α × List β
| [] => ([], [])
| (a, b) :: t => match unzip t with | (al, bl) => (a::al, b::bl)
/--
`O(n)`. `range n` is the numbers from `0` to `n` exclusive, in increasing order.
* `range 5 = [0, 1, 2, 3, 4]`
-/
def range (n : Nat) : List Nat :=
loop n []
where
loop : Nat → List Nat → List Nat
| 0, ns => ns
| n+1, ns => loop n (n::ns)
/--
`O(n)`. `iota n` is the numbers from `1` to `n` inclusive, in decreasing order.
* `iota 5 = [5, 4, 3, 2, 1]`
-/
def iota : Nat → List Nat
| 0 => []
| m@(n+1) => m :: iota n
/-- Tail-recursive version of `iota`. -/
def iotaTR (n : Nat) : List Nat :=
let rec go : Nat → List Nat → List Nat
| 0, r => r.reverse
| m@(n+1), r => go n (m::r)
go n []
@[csimp]
theorem iota_eq_iotaTR : @iota = @iotaTR :=
have aux (n : Nat) (r : List Nat) : iotaTR.go n r = r.reverse ++ iota n := by
induction n generalizing r with
| zero => simp [iota, iotaTR.go]
| succ n ih => simp [iota, iotaTR.go, ih, append_assoc]
funext fun n => by simp [iotaTR, aux]
/--
`O(|l|)`. `enumFrom n l` is like `enum` but it allows you to specify the initial index.
* `enumFrom 5 [a, b, c] = [(5, a), (6, b), (7, c)]`
-/
def enumFrom : Nat → List α → List (Nat × α)
| _, [] => nil
| n, x :: xs => (n, x) :: enumFrom (n + 1) xs
/--
`O(|l|)`. `enum l` pairs up each element with its index in the list.
* `enum [a, b, c] = [(0, a), (1, b), (2, c)]`
-/
def enum : List α → List (Nat × α) := enumFrom 0
/--
`O(|l|)`. `intersperse sep l` alternates `sep` and the elements of `l`:
* `intersperse sep [] = []`
* `intersperse sep [a] = [a]`
* `intersperse sep [a, b] = [a, sep, b]`
* `intersperse sep [a, b, c] = [a, sep, b, sep, c]`
-/
def intersperse (sep : α) : List α → List α
| [] => []
| [x] => [x]
| x::xs => x :: sep :: intersperse sep xs
/--
`O(|xs|)`. `intercalate sep xs` alternates `sep` and the elements of `xs`:
* `intercalate sep [] = []`
* `intercalate sep [a] = a`
* `intercalate sep [a, b] = a ++ sep ++ b`
* `intercalate sep [a, b, c] = a ++ sep ++ b ++ sep ++ c`
-/
def intercalate (sep : List α) (xs : List (List α)) : List α :=
join (intersperse sep xs)
/--
`bind xs f` is the bind operation of the list monad. It applies `f` to each element of `xs`
to get a list of lists, and then concatenates them all together.
* `[2, 3, 2].bind range = [0, 1, 0, 1, 2, 0, 1]`
-/
@[inline] protected def bind {α : Type u} {β : Type v} (a : List α) (b : α → List β) : List β := join (map b a)
/-- `pure x = [x]` is the `pure` operation of the list monad. -/
@[inline] protected def pure {α : Type u} (a : α) : List α := [a]
/--
The lexicographic order on lists.
`[] < a::as`, and `a::as < b::bs` if `a < b` or if `a` and `b` are equivalent and `as < bs`.
-/
inductive lt [LT α] : List α → List α → Prop where
/-- `[]` is the smallest element in the order. -/
| nil (b : α) (bs : List α) : lt [] (b::bs)
/-- If `a < b` then `a::as < b::bs`. -/
| head {a : α} (as : List α) {b : α} (bs : List α) : a < b → lt (a::as) (b::bs)
/-- If `a` and `b` are equivalent and `as < bs`, then `a::as < b::bs`. -/
| tail {a : α} {as : List α} {b : α} {bs : List α} : ¬ a < b → ¬ b < a → lt as bs → lt (a::as) (b::bs)
instance [LT α] : LT (List α) := ⟨List.lt⟩
instance hasDecidableLt [LT α] [h : DecidableRel (α:=α) (·<·)] : (l₁ l₂ : List α) → Decidable (l₁ < l₂)
| [], [] => isFalse (fun h => nomatch h)
| [], _::_ => isTrue (List.lt.nil _ _)
| _::_, [] => isFalse (fun h => nomatch h)
| a::as, b::bs =>
match h a b with
| isTrue h₁ => isTrue (List.lt.head _ _ h₁)
| isFalse h₁ =>
match h b a with
| isTrue h₂ => isFalse (fun h => match h with
| List.lt.head _ _ h₁' => absurd h₁' h₁
| List.lt.tail _ h₂' _ => absurd h₂ h₂')
| isFalse h₂ =>
match hasDecidableLt as bs with
| isTrue h₃ => isTrue (List.lt.tail h₁ h₂ h₃)
| isFalse h₃ => isFalse (fun h => match h with
| List.lt.head _ _ h₁' => absurd h₁' h₁
| List.lt.tail _ _ h₃' => absurd h₃' h₃)
/-- The lexicographic order on lists. -/
@[reducible] protected def le [LT α] (a b : List α) : Prop := ¬ b < a
instance [LT α] : LE (List α) := ⟨List.le⟩
instance [LT α] [DecidableRel ((· < ·) : α → α → Prop)] : (l₁ l₂ : List α) → Decidable (l₁ ≤ l₂) :=
fun _ _ => inferInstanceAs (Decidable (Not _))
/-- `isPrefixOf l₁ l₂` returns `true` Iff `l₁` is a prefix of `l₂`.
That is, there exists a `t` such that `l₂ == l₁ ++ t`. -/
def isPrefixOf [BEq α] : List α → List α → Bool
| [], _ => true
| _, [] => false
| a::as, b::bs => a == b && isPrefixOf as bs
/-- `isPrefixOf? l₁ l₂` returns `some t` when `l₂ == l₁ ++ t`. -/
def isPrefixOf? [BEq α] : List α → List α → Option (List α)
| [], l₂ => some l₂
| _, [] => none
| (x₁ :: l₁), (x₂ :: l₂) =>
if x₁ == x₂ then isPrefixOf? l₁ l₂ else none
/-- `isSuffixOf l₁ l₂` returns `true` Iff `l₁` is a suffix of `l₂`.
That is, there exists a `t` such that `l₂ == t ++ l₁`. -/
def isSuffixOf [BEq α] (l₁ l₂ : List α) : Bool :=
isPrefixOf l₁.reverse l₂.reverse
/-- `isSuffixOf? l₁ l₂` returns `some t` when `l₂ == t ++ l₁`.-/
def isSuffixOf? [BEq α] (l₁ l₂ : List α) : Option (List α) :=
Option.map List.reverse <| isPrefixOf? l₁.reverse l₂.reverse
/--
`O(min |as| |bs|)`. Returns true if `as` and `bs` have the same length,
and they are pairwise related by `eqv`.
-/
@[specialize] def isEqv : (as bs : List α) → (eqv : α → α → Bool) → Bool
| [], [], _ => true
| a::as, b::bs, eqv => eqv a b && isEqv as bs eqv
| _, _, _ => false
/--
The equality relation on lists asserts that they have the same length
and they are pairwise `BEq`.
-/
protected def beq [BEq α] : List α → List α → Bool
| [], [] => true
| a::as, b::bs => a == b && List.beq as bs
| _, _ => false
instance [BEq α] : BEq (List α) := ⟨List.beq⟩
/--
`replicate n a` is `n` copies of `a`:
* `replicate 5 a = [a, a, a, a, a]`
-/
@[simp] def replicate : (n : Nat) → (a : α) → List α
| 0, _ => []
| n+1, a => a :: replicate n a
/-- Tail-recursive version of `List.replicate`. -/
def replicateTR {α : Type u} (n : Nat) (a : α) : List α :=
let rec loop : Nat → List α → List α
| 0, as => as
| n+1, as => loop n (a::as)
loop n []
theorem replicateTR_loop_replicate_eq (a : α) (m n : Nat) :
replicateTR.loop a n (replicate m a) = replicate (n + m) a := by
induction n generalizing m with simp [replicateTR.loop]
| succ n ih => simp [Nat.succ_add]; exact ih (m+1)
@[csimp] theorem replicate_eq_replicateTR : @List.replicate = @List.replicateTR := by
apply funext; intro α; apply funext; intro n; apply funext; intro a
exact (replicateTR_loop_replicate_eq _ 0 n).symm
/--
Removes the last element of the list.
* `dropLast [] = []`
* `dropLast [a] = []`
* `dropLast [a, b, c] = [a, b]`
-/
def dropLast {α} : List α → List α
| [] => []
| [_] => []
| a::as => a :: dropLast as
@[simp] theorem length_replicate (n : Nat) (a : α) : (replicate n a).length = n := by
induction n <;> simp_all
@[simp] theorem length_concat (as : List α) (a : α) : (concat as a).length = as.length + 1 := by
induction as with
| nil => rfl
| cons _ xs ih => simp [concat, ih]
@[simp] theorem length_set (as : List α) (i : Nat) (a : α) : (as.set i a).length = as.length := by
induction as generalizing i with
| nil => rfl
| cons x xs ih =>
cases i with
| zero => rfl
| succ i => simp [set, ih]
@[simp] theorem length_dropLast_cons (a : α) (as : List α) : (a :: as).dropLast.length = as.length := by
match as with
| [] => rfl
| b::bs =>
have ih := length_dropLast_cons b bs
simp[dropLast, ih]
@[simp] theorem length_append (as bs : List α) : (as ++ bs).length = as.length + bs.length := by
induction as with
| nil => simp
| cons _ as ih => simp [ih, Nat.succ_add]
@[simp] theorem length_map (as : List α) (f : α → β) : (as.map f).length = as.length := by
induction as with
| nil => simp [List.map]
| cons _ as ih => simp [List.map, ih]
@[simp] theorem length_reverse (as : List α) : (as.reverse).length = as.length := by
induction as with
| nil => rfl
| cons a as ih => simp [ih]
/--
Returns the largest element of the list, if it is not empty.
* `[].maximum? = none`
* `[4].maximum? = some 4`
* `[1, 4, 2, 10, 6].maximum? = some 10`
-/
def maximum? [Max α] : List α → Option α
| [] => none
| a::as => some <| as.foldl max a
/--
Returns the smallest element of the list, if it is not empty.
* `[].minimum? = none`
* `[4].minimum? = some 4`
* `[1, 4, 2, 10, 6].minimum? = some 1`
-/
def minimum? [Min α] : List α → Option α
| [] => none
| a::as => some <| as.foldl min a
instance [BEq α] [LawfulBEq α] : LawfulBEq (List α) where
eq_of_beq {as bs} := by
induction as generalizing bs with
| nil => intro h; cases bs <;> first | rfl | contradiction
| cons a as ih =>
cases bs with
| nil => intro h; contradiction
| cons b bs =>
simp [show (a::as == b::bs) = (a == b && as == bs) from rfl]
intro ⟨h₁, h₂⟩
exact ⟨h₁, ih h₂⟩
rfl {as} := by
induction as with
| nil => rfl
| cons a as ih => simp [BEq.beq, List.beq, LawfulBEq.rfl]; exact ih
theorem of_concat_eq_concat {as bs : List α} {a b : α} (h : as.concat a = bs.concat b) : as = bs ∧ a = b := by
match as, bs with
| [], [] => simp [concat] at h; simp [h]
| [_], [] => simp [concat] at h
| _::_::_, [] => simp [concat] at h
| [], [_] => simp [concat] at h
| [], _::_::_ => simp [concat] at h
| _::_, _::_ => simp [concat] at h; simp [h]; apply of_concat_eq_concat h.2
theorem concat_eq_append (as : List α) (a : α) : as.concat a = as ++ [a] := by
induction as <;> simp [concat, *]
theorem drop_eq_nil_of_le {as : List α} {i : Nat} (h : as.length ≤ i) : as.drop i = [] := by
match as, i with
| [], i => simp
| _::_, 0 => simp at h
| _::as, i+1 => simp at h; exact @drop_eq_nil_of_le as i (Nat.le_of_succ_le_succ h)
end List