25 lines
705 B
Text
25 lines
705 B
Text
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--| Returns the first non-empty `Optional` value in a `List`
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let head
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: ∀(a : Type) → List (Optional a) → Optional a
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= λ(a : Type) →
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λ(xs : List (Optional a)) →
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List/fold
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(Optional a)
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xs
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(Optional a)
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( λ(l : Optional a) →
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λ(r : Optional a) →
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merge { Some = λ(x : a) → Some x, None = r } l
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)
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(None a)
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let example0 = assert : head Natural [ None Natural, Some 1, Some 2 ] ≡ Some 1
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let example1 =
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assert : head Natural [ None Natural, None Natural ] ≡ None Natural
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let example2 =
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assert : head Natural ([] : List (Optional Natural)) ≡ None Natural
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in head
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