September Morn Update

Task/Amb/Haskell/amb-2.hs

@@ -0,0 +1,53 @@
+joins :: String -> String -> Bool
+joins left right = last left == head right
+
+-- First desugaring (dropping the do notation)
+-- in terms of the bind operator (>>=) for the list monad
+
+exampleBind :: String
+exampleBind =
+  ["the", "that", "a"] >>=
+  (\w1 ->
+      ["frog", "elephant", "thing"] >>=
+      \w2 ->
+         ["walked", "treaded", "grows"] >>=
+         \w3 ->
+            ["slowly", "quickly"] >>=
+            (\w4 ->
+                if joins w1 w2
+                  then (if joins w2 w3
+                          then (if joins w3 w4
+                                  then unwords [w1, w2, w3, w4]
+                                  else [])
+                          else [])
+                  else []))
+
+-- Second desugaring (still dropping the do notation)
+-- in terms of the concatMap, which is >>= with its arguments flipped
+
+exampleConcatMap :: String
+exampleConcatMap =
+  concatMap
+    (\w1 ->
+        concatMap
+          (\w2 ->
+              concatMap
+                (\w3 ->
+                    concatMap
+                      (\w4 ->
+                          if joins w1 w2
+                            then (if joins w2 w3
+                                    then (if joins w3 w4
+                                            then unwords [w1, w2, w3, w4]
+                                            else [])
+                                    else [])
+                            else [])
+                      ["slowly", "quickly"])
+                ["walked", "treaded", "grows"])
+          ["frog", "elephant", "thing"])
+    ["the", "that", "a"]
+
+main :: IO ()
+main = do
+  print exampleBind
+  print exampleConcatMap

Task/Amb/Haskell/amb-3.hs

@@ -0,0 +1,22 @@
+example :: [String]
+example =
+  ["the", "that", "a"] >>=
+  \w1 ->
+     when True ["frog", "elephant", "thing"] >>=
+     \w2 ->
+        when (joins w1 w2) ["walked", "treaded", "grows"] >>=
+        \w3 ->
+           when (joins w2 w3) ["slowly", "quickly"] >>=
+           \w4 -> when (joins w3 w4) [w1, w2, w3, w4]
+
+joins :: String -> String -> Bool
+joins left right = last left == head right
+
+when :: Bool -> [a] -> [a]
+when p xs =
+  if p
+    then xs
+    else []
+
+main :: IO ()
+main = print $ unwords example

Task/Amb/Haskell/amb-4.hs

@@ -0,0 +1,16 @@
+joins :: String -> String -> Bool
+joins left right = last left == head right
+
+example :: [String]
+example =
+  [ unwords [w1, w2, w3, w4]
+  | w1 <- ["the", "that", "a"]
+  , w2 <- ["frog", "elephant", "thing"]
+  , w3 <- ["walked", "treaded", "grows"]
+  , w4 <- ["slowly", "quickly"]
+  , joins w1 w2
+  , joins w2 w3
+  , joins w3 w4 ]
+
+main :: IO ()
+main = print example