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43 lines
1.3 KiB
Idris
43 lines
1.3 KiB
Idris
module Data.List.Extra
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import Data.List
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%default total
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||| Analogous to `map` for `List`, but the function is applied to the index of
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||| the element as first argument (counting from 0), and the element itself as
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||| second argument.
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public export
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mapi : ((n : Nat) -> (a -> b))
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-> (xs : List a)
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-> List b
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mapi = h 0 where
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h : Nat -> ((n : Nat) -> (a -> b))
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-> (xs : List a)
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-> List b
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h i f [] = []
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h i f (x :: xs) = f i x :: h (S i) f xs
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||| Applied to a predicate and a list, returns the list of those elements that
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||| satisfy the predicate with corresponding indices in a stand-alone list.
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||| See also `Data.List.Extra.filterLoc'`.
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public export
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filterLoc : (a -> Bool) -> List a -> (List a, List Nat)
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filterLoc p = h 0 where
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h : Nat -> List a -> (List a, List Nat)
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h _ [] = ([], [])
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h lvl (x :: xs) = if p x
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then let (ms, ns) = h (S lvl) xs in (x :: ms, lvl :: ns)
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else h (S lvl) xs
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||| Applied to a predicate and a list, returns the list of those elements that
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||| satisfy the predicate with corresponding indices.
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public export
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filterLoc' : (a -> Bool) -> List a -> List (a, Nat)
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filterLoc' p = h 0 where
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h : Nat -> List a -> List (a, Nat)
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h _ [] = []
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h lvl (x :: xs) = if p x
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then (x, lvl) :: h (S lvl) xs
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else h (S lvl) xs
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