36 lines
943 B
Text
36 lines
943 B
Text
structure Date where
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val : Nat
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deriving Repr
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instance : LE Date := ⟨InvImage (Nat.le) Date.val⟩
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instance bad (a b : Date) : Decidable (a <= b) :=
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if h0 : (a.val <= b.val) then isTrue h0 else isFalse (fun hf => False.elim (h0 hf))
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instance : Min Date := minOfLe
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/-
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This implementation also fails:
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instance (a b : Date) : Decidable (a <= b) :=
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inferInstanceAs (Decidable (a.val <= b.val))
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-/
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/-
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This implementation evaluates successfully:
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instance (a b : Date) : Decidable (a <= b) :=
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dite (a.val <= b.val) isTrue (fun nle => isFalse (fun hf => False.elim (nle hf)))
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-/
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instance : ToString Date where
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toString d := s!"D{d.val}"
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structure DateRange where
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start_ : Date
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finish_ : Date
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h : start_ <= finish_
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def r1 := (DateRange.mk (Date.mk 0) (Date.mk 10) (by decide))
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-- If you change the start_ value here to `0`, it works.
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def r2 := (DateRange.mk (Date.mk 1) (Date.mk 10) (by decide))
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#eval min r1.start_ r2.start_
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