refactor: Foldable => Traversable

src/Init/Control/Traversable.lean

@@ -7,14 +7,13 @@ prelude
 import Init.Core
 
 /-
-  Typeclass for the polymorphic `foldlM` operation described in the paper.
+  Typeclass for the polymorphic `forM` operation described in the "do unchained" paper.
   Remark:
   - `γ` is a "container" type of elements of type `α`.
   - `α` is treated as an output parameter by the typeclass resolution procedure.
     That is, it tries to find an instance using only `m` and `γ`.
 -/
-class Foldable (m : Type u → Type v) (γ : Type w₁) (α : outParam (Type w₂)) where
-  foldlM [Monad m] : (δ → α → m δ) → δ → γ → m δ
+class Traversable (m : Type u → Type v) (γ : Type w₁) (α : outParam (Type w₂)) where
+  forM [Monad m] : γ → (α → m PUnit) → m PUnit
 
--- Add the alias `foldlM` for `Foldable.foldlM`
-export Foldable (foldlM)
+export Traversable (forM)

src/Init/Data/List/Control.lean

@@ -5,7 +5,7 @@ Author: Leonardo de Moura
 -/
 prelude
 import Init.Control.Basic
-import Init.Control.Foldable
+import Init.Control.Traversable
 import Init.Data.List.Basic
 
 namespace List
@@ -52,14 +52,16 @@ def mapA {m : Type u → Type v} [Applicative m] {α : Type w} {β : Type u} (f
   | a::as => List.cons <$> f a <*> mapA f as
 
 @[specialize]
-def forM {m : Type u → Type v} [Monad m] {α : Type w} (f : α → m PUnit) : List α → m PUnit
-  | []     => pure ⟨⟩
-  | h :: t => do f h; forM f t
+protected def forM {m : Type u → Type v} [Monad m] {α : Type w} (as : List α) (f : α → m PUnit) : m PUnit :=
+  match as with
+  | []      => pure ⟨⟩
+  | a :: as => do f a; List.forM as f
 
 @[specialize]
-def forA {m : Type u → Type v} [Applicative m] {α : Type w} (f : α → m PUnit) : List α → m PUnit
-  | []     => pure ⟨⟩
-  | h :: t => f h *> forA f t
+def forA {m : Type u → Type v} [Applicative m] {α : Type w} (as : List α) (f : α → m PUnit) : m PUnit :=
+  match as with
+  | []      => pure ⟨⟩
+  | a :: as => f a *> forA as f
 
 @[specialize]
 def filterAuxM {m : Type → Type v} [Monad m] {α : Type} (f : α → m Bool) : List α → List α → m (List α)
@@ -150,14 +152,12 @@ def findSomeM? {m : Type u → Type v} [Monad m] {α : Type w} {β : Type u} (f
 instance : ForIn m (List α) α where
   forIn := List.forIn
 
-instance : Foldable m (List α) α where
-  foldlM := List.foldlM
+instance : Traversable m (List α) α where
+  forM := List.forM
 
--- Add two simplification lemmas for `foldlM` over lists
-@[simp] theorem foldlM_nil  [Monad m] (f : δ → α → m δ) (d : δ) : foldlM f d ([] : List α) = pure d :=
+@[simp] theorem forM_nil  [Monad m] (f : α → m PUnit) : forM [] f = pure ⟨⟩ :=
   rfl
-
-@[simp] theorem foldlM_cons [Monad m] (f : δ → α → m δ) (d : δ) (a : α) (as : List α) : foldlM f d (a::as) = f d a >>= fun d => foldlM f d as :=
+@[simp] theorem forM_cons [Monad m] (f : α → m PUnit) (a : α) (as : List α) : forM (a::as) f = f a >>= fun _ => forM as f :=
   rfl
 
 end List

src/Init/Data/Range.lean

@@ -5,7 +5,7 @@ Authors: Leonardo de Moura
 -/
 prelude
 import Init.Meta
-import Init.Control.Foldable
+import Init.Control.Traversable
 
 namespace Std
 -- We put `Range` in `Init` because we want the notation `[i:j]`  without importing `Std`
@@ -32,17 +32,17 @@ universes u v
 instance : ForIn m Range Nat where
   forIn := Range.forIn
 
-@[inline] protected def foldlM {β : Type u} {m : Type u → Type v} [Monad m] (f : β → Nat → m β)  (init : β) (range : Range) : m β :=
-  let rec @[specialize] loop (i : Nat) (j : Nat) (b : β) : m β := do
+@[inline] protected def forM {m : Type u → Type v} [Monad m] (range : Range) (f : Nat → m PUnit) : m PUnit :=
+  let rec @[specialize] loop (i : Nat) (j : Nat) : m PUnit := do
     if j ≥ range.stop then
-      pure b
+      pure ⟨⟩
     else match i with
-     | 0   => pure b
-     | i+1 => loop i (j + range.step) (← f b j)
-  loop range.stop range.start init
+     | 0   => pure ⟨⟩
+     | i+1 => f j; loop i (j + range.step)
+  loop range.stop range.start
 
-instance : Foldable m Range Nat where
-  foldlM := Range.foldlM
+instance : Traversable m Range Nat where
+  forM := Range.forM
 
 syntax:max "[" ":" term "]" : term
 syntax:max "[" term ":" term "]" : term