[#222] CallStack attach

CHANGES.md

@@ -1,6 +1,9 @@
 Unreleased
 =====
 
+* [#222](https://github.com/serokell/universum/issues/222):
+  Attach `CallStack` to all functions from `Universum.Unsafe`.
+
 1.7.3
 =====
 

src/Universum/Unsafe.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE Unsafe #-}
+{-# LANGUAGE LambdaCase #-}
 
 {- | Unsafe functions to work with lists and 'Maybe'.
 Sometimes unavoidable but better don't use them. This module
@@ -15,28 +16,132 @@ foo = Unsafe.head
 -}
 
 module Universum.Unsafe
-       ( head
-       , tail
-       , init
-       , last
-       , at
-       , (!!)
-       , fromJust
-       , foldr1
-       , foldl1
-       , minimum
-       , maximum
-       , minimumBy
-       , maximumBy
-       ) where
-
-import Data.List (foldl1, foldr1, head, init, last, maximum, maximumBy, minimum, minimumBy, tail,
-                  (!!))
-import Data.Maybe (fromJust)
-
-import Universum.Base (Int)
+  ( head
+  , tail
+  , init
+  , last
+  , at
+  , (!!)
+  , fromJust
+  , foldr1
+  , foldl1
+  , minimum
+  , maximum
+  , minimumBy
+  , maximumBy
+  ) where
+
+import Data.Foldable (Foldable)
+import Data.List ((++), foldr, foldl, foldl1', foldl')
+import Data.Maybe (Maybe(..), fromJust, fromMaybe)
+import Data.Ord ((<), Ord (min, max), Ordering (..))
+import GHC.Base ((.), ($!))
+import GHC.Err (error)
+import GHC.Num ((-))
+
+import Universum.Base (HasCallStack, Int, String)
+
+-- | \(\mathcal{O}(1)\). Extract the first element of a list, which must be non-empty.
+head :: HasCallStack => [a] -> a
+head = \case
+  [] -> errorEmptyList "head"
+  (x : _) -> x
+
+-- | \(\mathcal{O}(1)\). Extract the elements after the head of a list, which
+-- must be non-empty.
+tail :: HasCallStack => [a] -> [a]
+tail = \case
+  [] -> errorEmptyList "tail"
+  (_ : xs) -> xs
+
+-- | \(\mathcal{O}(n)\). Return all the elements of a list except the last one.
+-- The list must be non-empty.
+init :: HasCallStack => [a] -> [a]
+init = \case
+  [_] -> []
+  (x : xs) -> x : init xs
+  [] -> errorEmptyList "init"
+
+-- | \(\mathcal{O}(n)\). Extract the last element of a list, which must be
+-- finite and non-empty.
+last :: HasCallStack => [a] -> a
+last = \case
+  [x] -> x
+  (_ : xs) -> last xs
+  [] -> errorEmptyList "last"
+
+-- | List index (subscript) operator, starting from 0.
+-- It is an instance of the more general 'Data.List.genericIndex',
+-- which takes an index of any integral type.
+(!!) :: HasCallStack => [a] -> Int -> a
+_      !! n | n < 0 =  error "Universum.!!: negative index"
+[]     !! _         =  error "Universum.!!: index too large"
+(x:_)  !! 0         =  x
+(_:xs) !! n         =  xs !! (n - 1)
+
+-- | A variant of 'foldr' that has no base case,
+-- and thus may only be applied to non-empty structures.
+--
+-- @'foldr1' f = 'List.foldr1' f . 'toList'@
+foldr1 :: (HasCallStack, Foldable t) => (a -> a -> a) -> t a -> a
+foldr1 f xs = fromMaybe (error "foldr1: empty structure") (foldr mf Nothing xs)
+  where
+    mf x m = Just (case m of
+                      Nothing -> x
+                      Just y  -> f x y)
+
+-- | A variant of 'foldl' that has no base case,
+-- and thus may only be applied to non-empty structures.
+--
+-- @'foldl1' f = 'List.foldl1' f . 'toList'@
+foldl1 :: (HasCallStack, Foldable t) => (a -> a -> a) -> t a -> a
+foldl1 f xs = fromMaybe (error "foldl1: empty structure") (foldl mf Nothing xs)
+  where
+    mf m y = Just (case m of
+                      Nothing -> y
+                      Just x  -> f x y)
+
+-- | The least element of a non-empty structure.
+minimum :: (HasCallStack, Ord a) => [a] -> a
+{-# INLINABLE minimum #-}
+minimum = \case
+  [] -> errorEmptyList "minimum"
+  xs -> foldl1' min xs
+
+-- | The largest element of a non-empty structure.
+maximum :: (HasCallStack, Ord a) => [a] -> a
+{-# INLINABLE maximum #-}
+maximum = \case
+  [] -> errorEmptyList "maximum"
+  xs -> foldl1' max xs
+
+-- | The least element of a non-empty structure with respect to the given comparison function.
+minimumBy :: (HasCallStack, Foldable t) => (a -> a -> Ordering) -> t a -> a
+minimumBy cmp = fromMaybe (error "minimumBy: empty structure") . foldl' min' Nothing
+  where
+    min' mx y = Just $! case mx of
+      Nothing -> y
+      Just x -> case cmp x y of
+        GT -> y
+        _ -> x
+{-# INLINEABLE minimumBy #-}
+
+-- | The largest element of a non-empty structure with respect to the given comparison function.
+maximumBy :: (HasCallStack, Foldable t) => (a -> a -> Ordering) -> t a -> a
+maximumBy cmp = fromMaybe (error "maximumBy: empty structure") . foldl' max' Nothing
+  where
+    max' mx y = Just $! case mx of
+      Nothing -> y
+      Just x -> case cmp x y of
+        GT -> x
+        _ -> y
+{-# INLINEABLE maximumBy #-}
 
 -- | Similar to '!!' but with flipped arguments.
 {-@ at :: n : Nat -> {xs : NonEmptyList a | len xs > n} -> a @-}
-at :: Int -> [a] -> a
+at :: HasCallStack => Int -> [a] -> a
 at n xs = xs !! n
+
+errorEmptyList :: HasCallStack => String -> a
+errorEmptyList fun =
+  error ("Universum." ++ fun ++ ": empty list")