lean2/tests/lean/run/eq20.lean

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import data.list
open nat list
context
parameter {A : Type}
parameter (p : A → Prop)
parameter [H : decidable_pred p]
include H
definition filter : list A → list A,
filter nil := nil,
filter (a :: l) := if p a then a :: filter l else filter l
theorem filter_nil : filter nil = nil :=
rfl
theorem filter_cons (a : A) (l : list A) : filter (a :: l) = if p a then a :: filter l else filter l :=
rfl
open eq.ops
theorem filter_cons_of_pos {a : A} (l : list A) (h : p a) : filter (a :: l) = a :: filter l :=
if_pos h ▸ filter_cons a l
theorem filter_cons_of_neg {a : A} (l : list A) (h : ¬ p a) : filter (a :: l) = filter l :=
if_neg h ▸ filter_cons a l
end
check @filter
check @filter_cons_of_pos
check @filter_cons_of_neg