24 lines
688 B
Text
24 lines
688 B
Text
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namespace Repro
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def FooM (α : Type) : Type := Unit → α
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def FooM.run {α : Type} (ψ : FooM α) (x : Unit) : α := ψ x
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def bind {α β : Type} : ∀ (ψ₁ : FooM α) (ψ₂ : α → FooM β), FooM β
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| ψ₁, ψ₂ => λ _ => ψ₂ (ψ₁.run ()) ()
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instance : Pure FooM := ⟨λ x => λ _ => x⟩
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instance : Bind FooM := ⟨@bind⟩
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instance : Monad FooM := {}
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def unexpectedBehavior : FooM String := do
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let b : Bool := (#[] : Array Nat).isEmpty;
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let trueBranch ← pure "trueBranch";
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let falseBranch ← pure "falseBranch";
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(1 : Nat).foldM (λ _ (s : String) => do
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let s ← pure $ if b then trueBranch else falseBranch; pure s) ""
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#eval unexpectedBehavior ()
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end Repro
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