From 4f67fe6cd314ad3ca1286766f1f9f3d8c5084d48 Mon Sep 17 00:00:00 2001
From: Adam Chlipala <adam@chlipala.net>
Date: Sun, 11 Dec 2011 15:02:55 -0500
Subject: Some new List functions, based on code by Ron de Bruijn

---
 lib/ur/list.ur  | 31 +++++++++++++++++++++++++++++++
 lib/ur/list.urs |  8 ++++++++
 2 files changed, 39 insertions(+)

(limited to 'lib')

diff --git a/lib/ur/list.ur b/lib/ur/list.ur
index 6a754287..2d5addc9 100644
--- a/lib/ur/list.ur
+++ b/lib/ur/list.ur
@@ -267,6 +267,18 @@ fun foldlMi [m] (_ : monad m) [a] [b] f =
         foldlMi' 0
     end
 
+fun filterM [m] (_ : monad m) [a] (p : a -> m bool) =
+    let
+        fun filterM' (acc : list a) (xs : list a) : m (list a) =
+            case xs of
+                [] => return (rev acc)
+              | x :: xs =>
+                c <- p x;
+                filterM' (if c then x :: acc else acc) xs
+    in
+        filterM' []
+    end
+
 fun all [m] f =
     let
         fun all' ls =
@@ -393,3 +405,22 @@ fun assocAdd [a] [b] (_ : eq a) (x : a) (y : b) (ls : t (a * b)) =
 fun recToList [a ::: Type] [r ::: {Unit}] (fl : folder r)
   = @foldUR [a] [fn _ => list a] (fn [nm ::_] [rest ::_] [[nm] ~ rest] x xs =>
 				      x :: xs) [] fl
+
+fun take [a] (n : int) (xs : list a) : list a = 
+    if n <= 0 then
+        []
+    else
+        case xs of
+            [] => []
+          | x :: xs => x :: take (n-1) xs
+
+fun drop [a] (n : int) (xs : list a) : list a =
+    if n <= 0 then
+        xs
+    else
+        case xs of
+            [] => []
+          | x :: xs => drop (n-1) xs
+
+fun splitAt [a] (n : int) (xs : list a) : list a * list a =
+    (take n xs, drop n xs)
diff --git a/lib/ur/list.urs b/lib/ur/list.urs
index 2a28d148..15204590 100644
--- a/lib/ur/list.urs
+++ b/lib/ur/list.urs
@@ -46,6 +46,9 @@ val foldlM : m ::: (Type -> Type) -> monad m -> a ::: Type -> b ::: Type
 val foldlMi : m ::: (Type -> Type) -> monad m -> a ::: Type -> b ::: Type
              -> (int -> a -> b -> m b) -> b -> t a -> m b
 
+val filterM : m ::: (Type -> Type) -> monad m -> a ::: Type
+              -> (a -> m bool) -> t a -> m (t a)
+
 val foldlMap : a ::: Type -> b ::: Type -> c ::: Type
                -> (a -> b -> c * b) -> b -> t a -> t c * b
 
@@ -90,3 +93,8 @@ val assocAdd : a ::: Type -> b ::: Type -> eq a -> a -> b -> t (a * b) -> t (a *
 (** Converting records to lists *)
 
 val recToList : a ::: Type -> r ::: {Unit} -> folder r -> $(mapU a r) -> t a
+
+(* Divide a list into two sections at a particular 0-based position, returning the second, first, or both parts, respectively. *)
+val drop : t ::: Type -> int -> list t -> list t
+val take : t ::: Type -> int -> list t -> list t
+val splitAt : t ::: Type -> int -> list t -> list t * list t
-- 
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