postgresql-syntax-0.2: library/PostgresqlSyntax/Extras/NonEmpty.hs

module PostgresqlSyntax.Extras.NonEmpty where

import PostgresqlSyntax.Prelude hiding (reverse, head, tail, init, last, cons, uncons, fromList)
import Data.List.NonEmpty


{-|
>>> intersperseFoldMap ", " id (fromList ["a"])
"a"

>>> intersperseFoldMap ", " id (fromList ["a", "b", "c"])
"a, b, c"
-}
intersperseFoldMap :: Monoid m => m -> (a -> m) -> NonEmpty a -> m
intersperseFoldMap a b (c :| d) = b c <> foldMap (mappend a . b) d

unsnoc :: NonEmpty a -> (Maybe (NonEmpty a), a)
unsnoc = let
  build1 = \ case
    a :| b -> build2 a b
  build2 a = \ case
    b : c -> build3 b (a :| []) c
    _ -> (Nothing, a)
  build3 a b = \ case
    c : d -> build3 c (cons a b) d
    _ -> (Just (reverse b), a)
  in build1

consAndUnsnoc :: a -> NonEmpty a -> (NonEmpty a, a)
consAndUnsnoc a b = case unsnoc b of
    (c, d) -> case c of
      Just e -> (cons a e, d)
      Nothing -> (pure a, d)