a108644461
Resolve cases when the `To/FromJSON` type classes are used with `Empty`, e.g. in the following motivating example. ``` import Lean structure Foo (α : Type) where y : Option α deriving Lean.ToJson #eval Lean.toJson (⟨none⟩ : Foo Empty) -- fails ``` This is a follow-up to this PR https://github.com/leanprover/lean4/pull/5415, as suggested by @eric-wieser. It expands on the original suggestion by also handling `FromJSON`. --------- Co-authored-by: Kyle Miller <kmill31415@gmail.com>
53 lines
1.6 KiB
Text
53 lines
1.6 KiB
Text
import Lean
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/-!
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# Validation of `Json Empty` instance
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This test serves a similar purpose to the `Repr Empty` test in `repr_empty.lean`.
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-/
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structure Foo (α : Type) where
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y : Option α
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deriving Lean.ToJson, Lean.FromJson, Repr
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/-! First, two basic examples with α not-Empty -/
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/-- info: {"y": 1} -/
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#guard_msgs in
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#eval Lean.toJson (⟨some 1⟩ : Foo Nat)
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/-! Ensure we can parse the type back -/
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/-- info: Except.ok { y := some 1 } -/
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#guard_msgs in
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#eval Lean.fromJson? (α := Foo Nat) <| json% {"y": 1}
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/-- info: {"y": null} -/
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#guard_msgs in
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#eval Lean.toJson (⟨none⟩ : Foo Nat)
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/-- info: Except.ok { y := none } -/
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#guard_msgs in
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#eval Lean.fromJson? (α := Foo Nat) <| json% {"y": null}
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/-! Examples with the `Empty` type -/
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/-- info: {"y": null} -/
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#guard_msgs in
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#eval Lean.toJson (⟨none⟩ : Foo Empty)
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/-! Show that we can round-trip from string -/
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/-- info: Except.ok { y := none } -/
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#guard_msgs in
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#eval Lean.fromJson? (α := Foo Empty) <| json% {"y": null}
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/-! Show that parsing fails -/
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/-- info: Except.error "type Empty has no constructor to match JSON value '\"Yo!\"'.
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This occurs when deserializing a value for type Empty, e.g. at type Option Empty with code
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for the 'some' constructor." -/
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#guard_msgs in
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#eval Lean.fromJson? (α := Empty) <| json% "Yo!"
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/-! Show that parsing fails if we supply anything else but `null` -/
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/-- info: Except.error "Foo.y: type Empty has no constructor to match JSON value '1'.
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This occurs when deserializing a value for type Empty, e.g. at type Option Empty with code
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for the 'some' constructor." -/
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#guard_msgs in
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#eval Lean.fromJson? (α := Foo Empty) <| json% {"y": 1}
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