3a309ba4eb
This PR improves the error message encountered in the case of a type class instance resolution failure, and adds an error explanation that discusses the common new-user case of binary operation overloading and points to the `trace.Meta.synthInstance` option for advanced debugging. ## Example ```lean4 def f (x : String) := x + x ``` Before: ``` failed to synthesize HAdd String String ?m.5 Hint: Additional diagnostic information may be available using the `set_option diagnostics true` command. ``` After: ``` failed to synthesize instance of type class HAdd String String ?m.5 Hint: Type class instance resolution failures can be inspected with the `set_option trace.Meta.synthInstance true` command. Error code: lean.failedToSynthesizeTypeclassInstance [View explanation](https://lean-lang.org/doc/reference/latest/find/?domain=Manual.errorExplanation&name=lean.failedToSynthesizeTypeclassInstance) ``` The error message is changed in three important ways: * Explains *what* failed to synthesize, using the "type class" terminology that's more likely to be recognized than the "instance" terminology * Points to the `trace.Meta.synthInstance` option which is otherwise nearly undiscoverable but is quite powerful (see also leanprover/reference-manual#663 which is adding commentary on this option) * Gives an error explanation link (which won't actually work until the next release after this is merged) which prioritizes the common-case explanation of using the wrong binary operation
29 lines
1 KiB
Text
29 lines
1 KiB
Text
class Fintype (α : Type u) where
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class Preorder (α : Type u) extends LT α, LE α where
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lt := fun a b => a ≤ b ∧ ¬b ≤ a
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structure Mappish (dIn dOut : Type u) [Fintype dIn] [Fintype dOut] where
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k : Nat
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variable {dIn dOut dOut₂ : Type} [Fintype dIn] [Fintype dOut] [Fintype dOut₂]
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def IsGood [DecidableEq dOut] [DecidableEq dOut₂] (Λ : Mappish dIn dOut) (Λ₂ : Mappish dIn dOut₂) : Prop :=
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∃ (D : Mappish dOut (dOut₂)), D.k = Λ.k + Λ₂.k
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/--
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error: failed to synthesize instance of type class
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Fintype v
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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 in
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def MappishOrder [DecidableEq dIn] : Preorder
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(Σ dOut : Sigma (fun t ↦ Fintype t × DecidableEq t), let fin := dOut.snd.1; Mappish dIn dOut.fst) where
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le Λ₁ Λ₂ := by
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let u := Λ₁.fst.fst;
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let v := Λ₂.fst.fst;
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let ⟨w,x⟩ := Λ₁.fst.snd;
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let ⟨y,z⟩ := Λ₂.fst.snd;
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clear y;
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exact @IsGood dIn v u _ _ _ _ _ Λ₂.snd Λ₁.snd
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