From ecc1d48c01ef5633d6229dc9009a899141b2eec7 Mon Sep 17 00:00:00 2001
From: defanor <defanor@uberspace.net>
Date: Sat, 2 Dec 2017 16:55:09 +0300
Subject: Add special handling for named Maybe values

Allow to omit named options.
---
 Coalpit.hs | 19 +++++++++++++++++++
 Example.hs |  6 +++---
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/Coalpit.hs b/Coalpit.hs
index c1d6380..ab7edff 100644
--- a/Coalpit.hs
+++ b/Coalpit.hs
@@ -11,6 +11,7 @@ import Data.List
 import GHC.Generics
 import Text.Megaparsec
 import Text.Megaparsec.Char
+import Data.Maybe
 
 type Parser = Parsec String String
 
@@ -53,6 +54,15 @@ instance (GToArgs a, GToArgs b) => GToArgs (a :+: b) where
   gToArgs c@(L1 x) = gToArgs x
   gToArgs c@(R1 x) = gToArgs x
 
+instance {-#OVERLAPPING#-}
+  (ArgParser a, Selector c) => GArgParser (S1 c (Rec0 (Maybe a))) where
+  gArgParser = do
+    case selName (undefined :: S1 c (Rec0 (Maybe a)) f) of
+           "" -> M1 <$> gArgParser
+           name -> do
+             x <- optional $ string ("--" ++ name) *> space *> argParser
+             pure $ M1 $ K1 x
+
 instance (GArgParser a, Selector c) => GArgParser (S1 c a) where
   gArgParser = M1 <$> do
     let sname = case selName (undefined :: S1 c a f) of
@@ -61,6 +71,13 @@ instance (GArgParser a, Selector c) => GArgParser (S1 c a) where
     sname *> gArgParser
 
 -- record selectors
+instance {-#OVERLAPPING#-}
+  (ToArgs a, Selector c) => GToArgs (S1 c (Rec0 (Maybe a))) where
+  gToArgs s@(M1 (K1 x)) = case (selName s, x) of
+                       ("", _) -> toArgs x
+                       (_, Nothing) -> []
+                       (name, Just x') -> ("--" ++ selName s) : toArgs x'
+
 instance (GToArgs a, Selector c) => GToArgs (S1 c a) where
   gToArgs s@(M1 x) = case selName s of
                          "" -> gToArgs x
@@ -104,3 +121,5 @@ instance ArgParser Double where
 instance ToArgs Double where
   toArgs i = [show i]
 
+instance ArgParser a => ArgParser (Maybe a)
+instance ToArgs a => ToArgs (Maybe a)
diff --git a/Example.hs b/Example.hs
index 725f8cc..a9f50aa 100644
--- a/Example.hs
+++ b/Example.hs
@@ -6,7 +6,7 @@ import GHC.Generics
 import Text.Megaparsec
 import Coalpit
 
-data RecTest = RecTest { a :: Int, b :: Double }
+data RecTest = RecTest { a :: Maybe Int, b :: Maybe Double, c :: Maybe Int }
              deriving (Generic, Show)
 
 instance ArgParser RecTest
@@ -14,7 +14,7 @@ instance ToArgs RecTest
 
 data Foo = Bar Int
          | Baz Int
-         | Qux RecTest
+         | Qux (Maybe Int) (Maybe Int) RecTest (Maybe Double)
   deriving (Generic, Show)
 
 instance ToArgs Foo
@@ -22,7 +22,7 @@ instance ArgParser Foo
 
 main :: IO ()
 main = do
-  let val = Qux (RecTest 1 2.3)
+  let val = Qux Nothing (Just 1) (RecTest Nothing (Just 2.3) Nothing) Nothing
       a = args val
   print val
   putStrLn a
-- 
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