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
71 lines
2.6 KiB
Text
71 lines
2.6 KiB
Text
/-
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Copyright (c) 2025 Lean FRO, LLC. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Authors: Joseph Rotella
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-/
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module
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prelude
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public import Lean.ErrorExplanation
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meta import Lean.ErrorExplanation
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public section
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/--
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This error occurs when an inductive type constructor is partially applied in the type of one of its
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constructors such that one or more parameters of the type are omitted. The elaborator requires that
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all parameters of an inductive type be specified everywhere that type is referenced in its
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definition, including in the types of its constructors.
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If it is necessary to allow the type constructor to be partially applied, without specifying a given
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type parameter, that parameter must be converted to an index. See the manual section on
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[Inductive Types](lean-manual://section/inductive-types) for further explanation of the difference
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between indices and parameters.
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# Examples
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## Omitting Parameter in Argument to Higher-Order Predicate
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```lean broken
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inductive List.All {α : Type u} (P : α → Prop) : List α → Prop
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| nil : All P []
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| cons {x xs} : P x → All P xs → All P (x :: xs)
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structure RoseTree (α : Type u) where
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val : α
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children : List (RoseTree α)
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inductive RoseTree.All {α : Type u} (P : α → Prop) (t : RoseTree α) : Prop
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| intro : P t.val → List.All (All P) t.children → All P t
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```
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```output
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Missing parameter(s) in occurrence of inductive type: In the expression
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List.All (All P) t.children
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found
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All P
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but expected all parameters to be specified:
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All P t
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Note: All occurrences of an inductive type in the types of its constructors must specify its fixed parameters. Only indices can be omitted in a partial application of the type constructor.
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```
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```lean fixed
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inductive List.All {α : Type u} (P : α → Prop) : List α → Prop
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| nil : All P []
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| cons {x xs} : P x → All P xs → All P (x :: xs)
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structure RoseTree (α : Type u) where
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val : α
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children : List (RoseTree α)
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inductive RoseTree.All {α : Type u} (P : α → Prop) : RoseTree α → Prop
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| intro : P t.val → List.All (All P) t.children → All P t
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```
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Because the `RoseTree.All` type constructor must be partially applied in the argument to `List.All`,
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the unspecified argument (`t`) must not be a parameter of the `RoseTree.All` predicate. Making it an
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index to the right of the colon in the header of `RoseTree.All` allows this partial application to
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succeed.
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-/
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register_error_explanation lean.inductiveParamMissing {
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summary := "Parameter not present in an occurrence of an inductive type in one of its constructors."
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sinceVersion := "4.22.0"
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}
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