lenary · October 10, 2017 20:40 · Oct 10, 2017 · Oct 10, 2017
diff --git a/test.hs b/test.hs
@@ -12,6 +12,12 @@ f xs =
             Left s -> Left s
             Right (res3, xs''') ->
               Right (B res1 res2 res3, xs''')
+
+-- This is exactly `Monad (Either String)`
+f' xs = do (res1, xs') <- f1 xs
+           (res2, xs'') <- f2 xs'
+           (res3, xs''') <- f3 xs''
+           return (B res1 res2 res3, xs''')
 
 
 -- In `g` we want to keep trying different functions with the same input
@@ -26,4 +32,7 @@ g xs =
         Left s ->
           case g3 xs of
             Right (res3, xs') -> Right (res3, xs')
-            Left s -> Left s
+            Left s -> Left s
+
+-- this is more complicated, lemme remember what it's called, but it's also possible.
+-- Maybe this is `Alternate (Either String)` but I cannot remember the name immediately.
diff --git a/test.hs b/test.hs
@@ -0,0 +1,29 @@
+-- In `f` we want to keep piping (part of) the result of one function into
+-- another, and stop as soon as something fails
+f :: [A] -> Either String (B, [A])
+f xs =
+  case f1 xs of
+    Left s -> Left s
+    Right (res1, xs') ->
+      case f2 xs' of
+        Left s -> Left s
+        Right (res2, xs'') ->
+          case f3 xs'' of
+            Left s -> Left s
+            Right (res3, xs''') ->
+              Right (B res1 res2 res3, xs''')
+
+
+-- In `g` we want to keep trying different functions with the same input
+-- until something succeeds, and then stop
+g :: [A] -> Either String (B, [A])
+g xs =
+  case g1 xs of
+    Right (res1, xs') -> Right (res1, xs')
+    Left s ->
+      case g2 xs of
+        Right (res2, xs') -> Right (res2, xs')
+        Left s ->
+          case g3 xs of
+            Right (res3, xs') -> Right (res3, xs')
+            Left s -> Left s
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