total (empty) → 1.0.0

raw patch · 4 files changed

+206/−0 lines, 4 filesdep +basedep +ghc-primdep +voidsetup-changed

Dependencies added: base, ghc-prim, void

Files

LICENSE +24/−0
Setup.hs +2/−0
src/Lens/Family/Total.hs +153/−0
total.cabal +27/−0

+ LICENSE view

@@ -0,0 +1,24 @@+Copyright (c) 2015 Gabriel Gonzalez+All rights reserved.++Redistribution and use in source and binary forms, with or without modification,+are permitted provided that the following conditions are met:+    * Redistributions of source code must retain the above copyright notice,+      this list of conditions and the following disclaimer.+    * Redistributions in binary form must reproduce the above copyright notice,+      this list of conditions and the following disclaimer in the documentation+      and/or other materials provided with the distribution.+    * Neither the name of Gabriel Gonzalez nor the names of other contributors+      may be used to endorse or promote products derived from this software+      without specific prior written permission.++THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR+ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON+ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

+ Setup.hs view

@@ -0,0 +1,2 @@+import Distribution.Simple+main = defaultMain

+ src/Lens/Family/Total.hs view

@@ -0,0 +1,153 @@+{-# LANGUAGE DefaultSignatures #-}+{-# LANGUAGE FlexibleContexts  #-}+{-# LANGUAGE TypeOperators     #-}++-- | This module lets you exhaustively pattern match on types using+-- `Lens.Family.Lens`es, `Lens.Family.Traversal`s, or `Lens.Family.Prism`s.+--+-- Here's an example use of this library:+--+-- > import Lens.Family.Total+-- > import Lens.Family.Stock+-- >+-- > total :: Either Char Int -> String       -- Same as:+-- > total = _case                            -- total = \case+-- >     & on _Left  (\c -> replicate 3  c )  --     Left  c -> replicate 3 c+-- >     & on _Right (\n -> replicate n '!')  --     Right n -> replicate n '!'+--+-- Our @example@ function pattern matches exhaustively on the `Either` type using+-- the `Lens.Family.Stock._Left` and `Lens.Family.Stock._Right` prisms:+--+-- >>> example (Left 'X')+-- "XXX"+-- >>> example (Right 2)+-- "!!"+--+-- The types ensure that the above function is total.  For example, if you omit+-- the `Lens.Family.Stock._Right` branch:+--+-- > partial :: Either Char Int -> String+-- > partial = _case+-- >     & on _Left  (\c -> replicate 3  c )+--+-- ... then you will get the following type error:+--+-- > No instance for (Empty Int) arising from a use of ‘_case’+-- > In the first argument of ‘(&)’, namely ‘_case’+-- > In the expression: _case & on _Left (\ c -> replicate 3 c)+-- > In an equation for ‘partial’:+-- >     partial = _case & on _Left (\ c -> replicate 3 c)+--+-- That type error means that you didn't pattern match on the branch with the+-- `Int`.+--+-- You can also implement exhaustive pattern matching for your own data types with+-- `Lens.Family.Traversal`s or `Control.Lens.Prism`s.  However, this only works if+-- you have one type variable for each branch of your type:+--+-- > {-# LANGUAGE DeriveGeneric   #-}+-- > {-# LANGUAGE TemplateHaskell #-}+-- >+-- > import Control.Lens.TH+-- > import GHC.Generics (Generic)+-- > import Lens.Family.Total+-- >+-- > data Example a b c = C1 a | C2 b | C3 c deriving (Generic)+-- >+-- > makePrisms ''Example+-- >+-- > instance (Empty a, Empty b, Empty c) => Empty (Example a b c)+-- >+-- > example :: Example String Char Int -> String  -- Same as:+-- > example = _case                               -- example = \case+-- >     & on _C1 (\s -> s              )          --     C1 s -> s+-- >     & on _C2 (\c -> replicate 3  c )          --     C2 c -> replicate 3  c+-- >     & on _C3 (\n -> replicate n '!')          --     C3 n -> replicate n '!'+--+-- There is no way to prove that the pattern match is exhaustive unless there is+-- a type parameter for every branch.  This is because each successive pattern+-- match `Void`s out that branch's type parameter to prove that the branch no+-- longer needs to be handled.  `_case` just verifies that all type parameters are+-- `Void`.+--+-- You can even pattern match using `Lens.Family.Lens`es, too:+--+-- > example :: (Int, Char) -> String     -- Same as:+-- > example = _case                      -- example = \case+-- >     & on _1 (\n -> replicate n '1')  --     (n, _) -> replicate n '1'+--+-- ... and of course the identity lens (`id`) works, too:+--+-- > example :: (Int, Char) -> String        -- Same as:+-- > example = _case                         -- example = \case+-- >     & on id (\(n, c) -> replicate n c)  --     (n, c) -> replicate n c++module Lens.Family.Total (+      Empty(..)+    , _case+    , on+    , (&)++    -- * Re-exports+    , Void+    ) where++import Data.Void (Void, absurd)+import GHC.Generics++-- | A type class for uninhabited types+class Empty a where+    impossible :: a -> x++    default impossible :: (Generic a, GEmpty (Rep a)) => a -> x+    impossible = gimpossible . from++instance Empty Void where+     impossible = absurd++instance (Empty a, Empty b) => Empty (Either a b) where+    impossible (Left  a) = impossible a+    impossible (Right b) = impossible b++instance Empty a => Empty ((,) a b) where+    impossible (a, _) = impossible a++class GEmpty f where+    gimpossible :: f a -> x++instance GEmpty V1 where+    gimpossible _ = undefined++instance (GEmpty a, GEmpty b) => GEmpty (a :+: b) where+    gimpossible (L1 l) = gimpossible l+    gimpossible (R1 r) = gimpossible r++instance Empty a => GEmpty (K1 i a) where+    gimpossible (K1 k) = impossible k++instance GEmpty a => GEmpty (M1 i c a) where+    gimpossible (M1 m) = gimpossible m++instance GEmpty a => GEmpty (a :*: b) where+    gimpossible (a :*: _) = gimpossible a++-- | Synonym for `impossible`, used to check if a pattern match is exhaustive+_case :: Empty a => a -> x+_case = impossible++-- | Pattern match on a `Lens.Family.Traversal`+on+    :: ((a -> Either a Void) -> i -> Either l r)+    -> (l -> o)+    -> (r -> o)+    -> i+    -> o+on p f g x = case p Left x of+    Left  l -> f l+    Right r -> g r++-- | Operator for post-fix function application+(&) :: a -> (a -> b) -> b+x & f = f x++infixl 1 &

+ total.cabal view

@@ -0,0 +1,27 @@+Name: total+Version: 1.0.0+Cabal-Version: >=1.8.0.2+Build-Type: Simple+License: BSD3+License-File: LICENSE+Copyright: 2015 Gabriel Gonzalez+Author: Gabriel Gonzalez+Maintainer: Gabriel439@gmail.com+Bug-Reports: https://github.com/Gabriel439/Haskell-Total-Library/issues+Synopsis: Exhaustive pattern matching using lenses, traversals, and prisms+Description: Use this library if you wish to hide your data type's internal+  representation while still providing a way to exhaustively pattern match on+  your type using exposed lenses, traversals, or prisms.+Category: Control+Source-Repository head+    Type: git+    Location: https://github.com/Gabriel439/Haskell-Total-Library++Library+    Hs-Source-Dirs: src+    Build-Depends:+        base     >= 4       && < 4.8,+        ghc-prim >= 0.2.0.0 && < 0.4,+        void     >= 0.4     && < 0.8+    Exposed-Modules: Lens.Family.Total+    GHC-Options: -O2 -Wall