chore(Data/Finsupp): split off basic scalar multiplication (#19205)

In the comments of #19095 we discussed that `MonoidAlgebra` and `Polynomial` need scalar multiplication by `Nat` or `Int` early on, to define a `Semiring` or `Ring` structure. We can define a generic `SMulWithZero` instance and then specialize it to get `nsmul` and `zsmul`, but only if that instance is available early on for `Finsupp`. So: split this off from `Finsupp/Basic.lean` into a higher-up file.

This PR used Damiano's `upstreamableDecls` linter to find out which results could move together with the definitions for free.

Mathlib.lean

@@ -2498,6 +2498,7 @@ import Mathlib.Data.Finsupp.Notation
 import Mathlib.Data.Finsupp.Order
 import Mathlib.Data.Finsupp.PWO
 import Mathlib.Data.Finsupp.Pointwise
+import Mathlib.Data.Finsupp.SMulWithZero
 import Mathlib.Data.Finsupp.ToDFinsupp
 import Mathlib.Data.Finsupp.Weight
 import Mathlib.Data.Finsupp.WellFounded

Mathlib/Data/Finsupp/Basic.lean

@@ -7,6 +7,7 @@ import Mathlib.Algebra.BigOperators.Finsupp
 import Mathlib.Algebra.Group.Action.Basic
 import Mathlib.Algebra.Module.Basic
 import Mathlib.Algebra.Regular.SMul
+import Mathlib.Data.Finsupp.SMulWithZero
 import Mathlib.Data.Rat.BigOperators
 
 /-!
@@ -1285,25 +1286,11 @@ end
 
 section
 
-instance smulZeroClass [Zero M] [SMulZeroClass R M] : SMulZeroClass R (α →₀ M) where
-  smul a v := v.mapRange (a • ·) (smul_zero _)
-  smul_zero a := by
-    ext
-    apply smul_zero
-
 /-!
 Throughout this section, some `Monoid` and `Semiring` arguments are specified with `{}` instead of
 `[]`. See note [implicit instance arguments].
 -/
 
-@[simp, norm_cast]
-theorem coe_smul [Zero M] [SMulZeroClass R M] (b : R) (v : α →₀ M) : ⇑(b • v) = b • ⇑v :=
-  rfl
-
-theorem smul_apply [Zero M] [SMulZeroClass R M] (b : R) (v : α →₀ M) (a : α) :
-    (b • v) a = b • v a :=
-  rfl
-
 theorem _root_.IsSMulRegular.finsupp [Zero M] [SMulZeroClass R M] {k : R}
     (hk : IsSMulRegular M k) : IsSMulRegular (α →₀ M) k :=
   fun _ _ h => ext fun i => hk (DFunLike.congr_fun h i)
@@ -1314,46 +1301,21 @@ instance faithfulSMul [Nonempty α] [Zero M] [SMulZeroClass R M] [FaithfulSMul R
     let ⟨a⟩ := ‹Nonempty α›
     eq_of_smul_eq_smul fun m : M => by simpa using DFunLike.congr_fun (h (single a m)) a
 
-instance instSMulWithZero [Zero R] [Zero M] [SMulWithZero R M] : SMulWithZero R (α →₀ M) where
-  zero_smul f := by ext i; exact zero_smul _ _
-
 variable (α M)
 
-instance distribSMul [AddZeroClass M] [DistribSMul R M] : DistribSMul R (α →₀ M) where
-  smul := (· • ·)
-  smul_add _ _ _ := ext fun _ => smul_add _ _ _
-  smul_zero _ := ext fun _ => smul_zero _
-
 instance distribMulAction [Monoid R] [AddMonoid M] [DistribMulAction R M] :
     DistribMulAction R (α →₀ M) :=
   { Finsupp.distribSMul _ _ with
     one_smul := fun x => ext fun y => one_smul R (x y)
     mul_smul := fun r s x => ext fun y => mul_smul r s (x y) }
 
-instance isScalarTower [Zero M] [SMulZeroClass R M] [SMulZeroClass S M] [SMul R S]
-  [IsScalarTower R S M] : IsScalarTower R S (α →₀ M) where
-  smul_assoc _ _ _ := ext fun _ => smul_assoc _ _ _
-
-instance smulCommClass [Zero M] [SMulZeroClass R M] [SMulZeroClass S M] [SMulCommClass R S M] :
-  SMulCommClass R S (α →₀ M) where
-  smul_comm _ _ _ := ext fun _ => smul_comm _ _ _
-
-instance isCentralScalar [Zero M] [SMulZeroClass R M] [SMulZeroClass Rᵐᵒᵖ M] [IsCentralScalar R M] :
-  IsCentralScalar R (α →₀ M) where
-  op_smul_eq_smul _ _ := ext fun _ => op_smul_eq_smul _ _
-
 instance module [Semiring R] [AddCommMonoid M] [Module R M] : Module R (α →₀ M) :=
   { toDistribMulAction := Finsupp.distribMulAction α M
     zero_smul := fun _ => ext fun _ => zero_smul _ _
     add_smul := fun _ _ _ => ext fun _ => add_smul _ _ _ }
 
 variable {α M}
 
-theorem support_smul [AddMonoid M] [SMulZeroClass R M] {b : R} {g : α →₀ M} :
-    (b • g).support ⊆ g.support := fun a => by
-  simp only [smul_apply, mem_support_iff, Ne]
-  exact mt fun h => h.symm ▸ smul_zero _
-
 @[simp]
 theorem support_smul_eq [Semiring R] [AddCommMonoid M] [Module R M] [NoZeroSMulDivisors R M] {b : R}
     (hb : b ≠ 0) {g : α →₀ M} : (b • g).support = g.support :=
@@ -1376,25 +1338,11 @@ theorem mapDomain_smul {_ : Monoid R} [AddCommMonoid M] [DistribMulAction R M] {
     (v : α →₀ M) : mapDomain f (b • v) = b • mapDomain f v :=
   mapDomain_mapRange _ _ _ _ (smul_add b)
 
-@[simp]
-theorem smul_single [Zero M] [SMulZeroClass R M] (c : R) (a : α) (b : M) :
-    c • Finsupp.single a b = Finsupp.single a (c • b) :=
-  mapRange_single
-
 -- Porting note: removed `simp` because `simpNF` can prove it.
 theorem smul_single' {_ : Semiring R} (c : R) (a : α) (b : R) :
     c • Finsupp.single a b = Finsupp.single a (c * b) :=
   smul_single _ _ _
 
-theorem mapRange_smul {_ : Monoid R} [AddMonoid M] [DistribMulAction R M] [AddMonoid N]
-    [DistribMulAction R N] {f : M → N} {hf : f 0 = 0} (c : R) (v : α →₀ M)
-    (hsmul : ∀ x, f (c • x) = c • f x) : mapRange f hf (c • v) = c • mapRange f hf v := by
-  erw [← mapRange_comp]
-  · have : f ∘ (c • ·) = (c • ·) ∘ f := funext hsmul
-    simp_rw [this]
-    apply mapRange_comp
-  simp only [Function.comp_apply, smul_zero, hf]
-
 theorem smul_single_one [Semiring R] (a : α) (b : R) : b • single a (1 : R) = single a b := by
   rw [smul_single, smul_eq_mul, mul_one]
 
@@ -1701,4 +1649,4 @@ end Sigma
 
 end Finsupp
 
-set_option linter.style.longFile 1900
+set_option linter.style.longFile 1700

Mathlib/Data/Finsupp/SMulWithZero.lean

@@ -0,0 +1,102 @@
+/-
+Copyright (c) 2017 Johannes Hölzl. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Johannes Hölzl, Kim Morrison
+-/
+import Mathlib.Algebra.Group.Action.Pi
+import Mathlib.Algebra.SMulWithZero
+import Mathlib.Data.Finsupp.Defs
+
+/-!
+# Scalar multiplication on `Finsupp`
+
+This file defines the pointwise scalar multiplication on `Finsupp`, assuming it preserves zero.
+
+## Main declarations
+
+* `Finsupp.smulZeroClass`: if the action of `R` on `M` preserves `0`, then it acts on `α →₀ M`
+
+## Implementation notes
+
+This file is intermediate between `Finsupp.Defs` and `Finsupp.Module` in that it covers scalar
+multiplication but does not rely on the definition of `Module`. Scalar multiplication is needed to
+supply the `nsmul` (and `zsmul`) fields of (semi)ring structures which are fundamental for e.g.
+`Polynomial`, so we want to keep the imports requied for the `Finsupp.smulZeroClass` instance
+reasonably light.
+
+This file is a `noncomputable theory` and uses classical logic throughout.
+-/
+
+assert_not_exists Module
+
+noncomputable section
+
+open Finset Function
+
+variable {α β γ ι M M' N P G H R S : Type*}
+
+namespace Finsupp
+
+instance smulZeroClass [Zero M] [SMulZeroClass R M] : SMulZeroClass R (α →₀ M) where
+  smul a v := v.mapRange (a • ·) (smul_zero _)
+  smul_zero a := by
+    ext
+    apply smul_zero
+
+/-!
+Throughout this section, some `Monoid` and `Semiring` arguments are specified with `{}` instead of
+`[]`. See note [implicit instance arguments].
+-/
+
+@[simp, norm_cast]
+theorem coe_smul [Zero M] [SMulZeroClass R M] (b : R) (v : α →₀ M) : ⇑(b • v) = b • ⇑v :=
+  rfl
+
+theorem smul_apply [Zero M] [SMulZeroClass R M] (b : R) (v : α →₀ M) (a : α) :
+    (b • v) a = b • v a :=
+  rfl
+
+instance instSMulWithZero [Zero R] [Zero M] [SMulWithZero R M] : SMulWithZero R (α →₀ M) where
+  zero_smul f := by ext i; exact zero_smul _ _
+
+variable (α M)
+
+instance distribSMul [AddZeroClass M] [DistribSMul R M] : DistribSMul R (α →₀ M) where
+  smul := (· • ·)
+  smul_add _ _ _ := ext fun _ => smul_add _ _ _
+  smul_zero _ := ext fun _ => smul_zero _
+
+instance isScalarTower [Zero M] [SMulZeroClass R M] [SMulZeroClass S M] [SMul R S]
+  [IsScalarTower R S M] : IsScalarTower R S (α →₀ M) where
+  smul_assoc _ _ _ := ext fun _ => smul_assoc _ _ _
+
+instance smulCommClass [Zero M] [SMulZeroClass R M] [SMulZeroClass S M] [SMulCommClass R S M] :
+  SMulCommClass R S (α →₀ M) where
+  smul_comm _ _ _ := ext fun _ => smul_comm _ _ _
+
+instance isCentralScalar [Zero M] [SMulZeroClass R M] [SMulZeroClass Rᵐᵒᵖ M] [IsCentralScalar R M] :
+  IsCentralScalar R (α →₀ M) where
+  op_smul_eq_smul _ _ := ext fun _ => op_smul_eq_smul _ _
+
+variable {α M}
+
+theorem support_smul [AddMonoid M] [SMulZeroClass R M] {b : R} {g : α →₀ M} :
+    (b • g).support ⊆ g.support := fun a => by
+  simp only [smul_apply, mem_support_iff, Ne]
+  exact mt fun h => h.symm ▸ smul_zero _
+
+@[simp]
+theorem smul_single [Zero M] [SMulZeroClass R M] (c : R) (a : α) (b : M) :
+    c • Finsupp.single a b = Finsupp.single a (c • b) :=
+  mapRange_single
+
+theorem mapRange_smul {_ : Monoid R} [AddMonoid M] [DistribMulAction R M] [AddMonoid N]
+    [DistribMulAction R N] {f : M → N} {hf : f 0 = 0} (c : R) (v : α →₀ M)
+    (hsmul : ∀ x, f (c • x) = c • f x) : mapRange f hf (c • v) = c • mapRange f hf v := by
+  erw [← mapRange_comp]
+  · have : f ∘ (c • ·) = (c • ·) ∘ f := funext hsmul
+    simp_rw [this]
+    apply mapRange_comp
+  simp only [Function.comp_apply, smul_zero, hf]
+
+end Finsupp