Add list difference to base Data.List module.

libs/base/Data/List.idr

@@ -206,6 +206,31 @@ public export
 delete : Eq a => a -> List a -> List a
 delete = deleteBy (==)
 
+||| Delete the first occurrence of each element from the second list in the first
+||| list where equality is determined by the predicate passed as the first argument.
+||| @ p            A function that returns true when its two arguments should be considered equal.
+||| @ source       The list to delete elements from.
+||| @ undesirables The list of elements to delete.
+public export
+deleteFirstsBy : (p : a -> a -> Bool) -> (source : List a) -> (undesirables : List a) -> List a
+deleteFirstsBy p = foldl (flip (deleteBy p))
+
+infix 7 \\
+
+||| The non-associative list-difference.
+||| A specialized form of @deleteFirstsBy@ where the predicate is equality under the @Eq@
+||| interface.
+||| Deletes the first occurrence of each element of the second list from the first list.
+|||
+||| In the following example, the result is `[2, 4]`.
+||| ```idris example
+||| [1, 2, 3, 4] // [1, 3]
+||| ```
+|||
+public export
+(\\) : (Eq a) => List a -> List a -> List a
+(\\) = foldl (flip delete)
+
 ||| The unionBy function returns the union of two lists by user-supplied equality predicate.
 public export
 unionBy : (a -> a -> Bool) -> List a -> List a -> List a