429734c02f
This PR modifies the error message for type synthesis failure for the case where the type class in question is potentially derivable using a `deriving` command. Also changes the error explanation for type class instance synthesis failure with an illustration of this pattern. ## Example ```lean4 inductive MyColor where | chartreuse | sienna | thistle def forceColor (oc : Option MyColor) := oc.get! ``` Before this PR, this gives the potentially confusing impression that Lean may have decided that `MyColor` is _not_ inhabited — people used to Rust may be especially inclined towards this confusion. ``` failed to synthesize instance of type class Inhabited MyColor Hint: Type class instance resolution failures can be inspected with the `set_option trace.Meta.synthInstance true` command. ``` After this PR, a targeted hint suggests precisely the command that will fix the issue: ``` error: failed to synthesize instance of type class Inhabited MyColor Hint: Adding the command `deriving instance Inhabited for MyColor` may allow Lean to derive the missing instance. ```
201 lines
5.3 KiB
Text
201 lines
5.3 KiB
Text
namespace RegularBEq
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set_option deriving.beq.linear_construction_threshold 1000
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-- set_option trace.Elab.Deriving.lawfulBEq true
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inductive L (α : Type u) where
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| nil : L α
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| cons : α → L α → L α
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deriving BEq, ReflBEq, LawfulBEq
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/--
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info: theorem RegularBEq.instReflBEqL.{u_1} : ∀ {α : Type u_1} [inst : BEq α] [ReflBEq α], ReflBEq (L α)
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-/
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#guard_msgs in
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#print sig instReflBEqL
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inductive Vec (α : Type u) : Nat → Type u where
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| nil : Vec α 0
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| cons : ∀ {n}, α → Vec α n → Vec α (n+1)
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deriving BEq, ReflBEq, LawfulBEq
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/--
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info: theorem RegularBEq.instReflBEqVec.{u_1} : ∀ {α : Type u_1} {a : Nat} [inst : BEq α] [ReflBEq α], ReflBEq (Vec α a)
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-/
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#guard_msgs in
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#print sig instReflBEqVec
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inductive Enum
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| mk1 | mk2 | mk3
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deriving BEq, ReflBEq, LawfulBEq
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/-- info: theorem RegularBEq.instReflBEqEnum : ReflBEq Enum -/
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#guard_msgs in
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#print sig instReflBEqEnum
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-- The following type has `Eq.rec`’s in its `BEq` implementation,
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-- but `simp` seems to handle that just fine
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inductive WithHEq (α : Type u) : Nat → Type u where
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| nil : WithHEq α 0
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| cons : ∀ {n m} , α → WithHEq α n → WithHEq α m → WithHEq α (n+1)
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deriving BEq, ReflBEq, LawfulBEq
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/--
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info: RegularBEq.instReflBEqWithHEq.{u_1} {α✝ : Type u_1} {a✝ : Nat} [BEq α✝] [ReflBEq α✝] : ReflBEq (WithHEq α✝ a✝)
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-/
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#guard_msgs in
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#check instReflBEqWithHEq
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/--
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info: RegularBEq.instLawfulBEqWithHEq.{u_1} {α✝ : Type u_1} {a✝ : Nat} [BEq α✝] [LawfulBEq α✝] : LawfulBEq (WithHEq α✝ a✝)
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-/
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#guard_msgs in
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#check instLawfulBEqWithHEq
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-- No `BEq` derived? Not a great error message yet, but the error location helps, so good enough.
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/--
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error: failed to synthesize instance of type class
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BEq Foo
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Hint: Adding the command `deriving instance BEq for RegularBEq.Foo` may allow Lean to derive the missing instance.
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-/
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#guard_msgs in
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structure Foo where
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deriving ReflBEq
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-- No `ReflBEq` but `LawfulBEq`? ot a great error message yet.
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/--
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@ +2:16...25
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error: failed to synthesize instance of type class
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ReflBEq Bar
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Hint: Type class instance resolution failures can be inspected with the `set_option trace.Meta.synthInstance true` command.
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-/
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#guard_msgs (positions := true) in
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structure Bar where
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deriving BEq, LawfulBEq
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/--
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@ +5:16...23
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error: Deriving `ReflBEq` for mutual inductives is not supported
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-/
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#guard_msgs (positions := true) in
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mutual
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inductive Tree (α : Type u) where
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| node : TreeList α → Tree α
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| leaf : α → Tree α
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deriving BEq, ReflBEq, LawfulBEq
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inductive TreeList (α : Type u) where
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| nil : TreeList α
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| cons : Tree α → TreeList α → TreeList α
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deriving BEq
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end
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end RegularBEq
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namespace LinearBEq
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set_option deriving.beq.linear_construction_threshold 0
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-- set_option trace.Elab.Deriving.lawfulBEq true
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inductive L (α : Type u) where
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| nil : L α
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| cons : α → L α → L α
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deriving BEq, ReflBEq, LawfulBEq
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/--
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info: theorem LinearBEq.instReflBEqL.{u_1} : ∀ {α : Type u_1} [inst : BEq α] [ReflBEq α], ReflBEq (L α)
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-/
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#guard_msgs in
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#print sig instReflBEqL
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inductive Vec (α : Type u) : Nat → Type u where
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| nil : Vec α 0
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| cons : ∀ {n}, α → Vec α n → Vec α (n+1)
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deriving BEq, ReflBEq, LawfulBEq
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/--
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info: theorem LinearBEq.instReflBEqVec.{u_1} : ∀ {α : Type u_1} {a : Nat} [inst : BEq α] [ReflBEq α], ReflBEq (Vec α a)
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-/
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#guard_msgs in
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#print sig instReflBEqVec
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inductive Enum
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| mk1 | mk2 | mk3
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deriving BEq, ReflBEq, LawfulBEq
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/-- info: theorem LinearBEq.instReflBEqEnum : ReflBEq Enum -/
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#guard_msgs in
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#print sig instReflBEqEnum
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-- The following type has `Eq.rec`’s in its `BEq` implementation,
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-- but `simp` seems to handle that just fine
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inductive WithHEq (α : Type u) : Nat → Type u where
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| nil : WithHEq α 0
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| cons : ∀ {n m} , α → WithHEq α n → WithHEq α m → WithHEq α (n+1)
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deriving BEq, ReflBEq, LawfulBEq
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/--
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info: LinearBEq.instReflBEqWithHEq.{u_1} {α✝ : Type u_1} {a✝ : Nat} [BEq α✝] [ReflBEq α✝] : ReflBEq (WithHEq α✝ a✝)
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-/
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#guard_msgs in
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#check instReflBEqWithHEq
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/--
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info: LinearBEq.instLawfulBEqWithHEq.{u_1} {α✝ : Type u_1} {a✝ : Nat} [BEq α✝] [LawfulBEq α✝] : LawfulBEq (WithHEq α✝ a✝)
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-/
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#guard_msgs in
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#check instLawfulBEqWithHEq
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-- No `BEq` derived? Not a great error message yet, but the error location helps, so good enough.
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/--
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error: failed to synthesize instance of type class
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BEq Foo
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Hint: Adding the command `deriving instance BEq for LinearBEq.Foo` may allow Lean to derive the missing instance.
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-/
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#guard_msgs in
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structure Foo where
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deriving ReflBEq
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-- No `ReflBEq` but `LawfulBEq`? ot a great error message yet.
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/--
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@ +2:16...25
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error: failed to synthesize instance of type class
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ReflBEq Bar
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Hint: Type class instance resolution failures can be inspected with the `set_option trace.Meta.synthInstance true` command.
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-/
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#guard_msgs (positions := true) in
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structure Bar where
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deriving BEq, LawfulBEq
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/--
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@ +5:16...23
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error: Deriving `ReflBEq` for mutual inductives is not supported
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-/
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#guard_msgs (positions := true) in
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mutual
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inductive Tree (α : Type u) where
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| node : TreeList α → Tree α
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| leaf : α → Tree α
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deriving BEq, ReflBEq, LawfulBEq
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inductive TreeList (α : Type u) where
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| nil : TreeList α
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| cons : Tree α → TreeList α → TreeList α
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deriving BEq
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end
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end LinearBEq
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