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. ```
96 lines
2.8 KiB
Text
96 lines
2.8 KiB
Text
import Lean
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open Lean
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inductive MyColor where
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| chartreuse | sienna | thistle
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/--
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error: failed to synthesize instance of type class
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Inhabited MyColor
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Hint: Adding the command `deriving instance Inhabited for MyColor` may allow Lean to derive the missing instance.
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-/
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#guard_msgs in
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def forceColor (oc : Option MyColor) :=
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oc.get!
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/--
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error: failed to synthesize instance of type class
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ToJson MyColor
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Hint: Adding the command `deriving instance Lean.ToJson for MyColor` may allow Lean to derive the missing instance.
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-/
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#guard_msgs in
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def jsonMaybeColor (oc : MyColor) :=
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ToJson.toJson oc
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/--
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error: failed to synthesize instance of type class
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ToJson MyColor
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Hint: Adding the command `deriving instance Lean.ToJson for MyColor` may allow Lean to derive the missing instance.
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---
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error: failed to synthesize instance of type class
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ToExpr MyColor
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Hint: Adding the command `deriving instance Lean.ToExpr for MyColor` may allow Lean to derive the missing instance.
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---
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error: No deriving handlers have been implemented for class `ToString`
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-/
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#guard_msgs in
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inductive MyList where
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| nil : MyList
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| cons : MyColor → MyList
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deriving ToJson, ToExpr, ToString, Nonempty
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-- It would be very cool if this case could give us "maybe derive ToJson MyColor", but that's more sophisticated than we can presently manage
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/--
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error: failed to synthesize instance of type class
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ToJson (Option MyColor)
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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 jsonColor (oc : Option MyColor) :=
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ToJson.toJson oc
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-- It would be very cool if this case could give us "maybe derive DecidableEq MyColor"
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/--
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error: failed to synthesize instance of type class
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Decidable (oc = some MyColor.thistle)
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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 checkColor (oc : Option MyColor) :=
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if oc = .some .thistle then 1 else 2
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inductive MyTree where
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| nil : MyTree
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| cons : MyColor → MyTree → MyTree
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-- The suggestion here will fail, it would be super awesome if we gave the full recommendation,
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-- but each hint makes progress
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/--
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error: failed to synthesize instance of type class
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ToJson MyTree
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Hint: Adding the command `deriving instance Lean.ToJson for MyTree` may allow Lean to derive the missing instance.
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-/
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#guard_msgs in
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def jsonListColor (oc : MyTree) :=
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ToJson.toJson oc
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/--
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error: failed to synthesize instance of type class
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ToJson MyColor
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Hint: Adding the command `deriving instance Lean.ToJson for MyColor` may allow Lean to derive the missing instance.
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-/
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#guard_msgs in
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deriving instance Lean.ToJson for MyTree
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deriving instance Lean.ToJson for MyColor
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deriving instance Lean.ToJson for MyTree
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