Progress

src/posts/0010-Haskell-Now/io_bind.hs

@@ -0,0 +1,17 @@
+import Data.Maybe
+import Text.Read (readMaybe)
+
+getListFromString :: String -> Maybe [Integer]
+getListFromString str = readMaybe $ "[" ++ str ++ "]"
+askUser :: IO [Integer]
+askUser =
+    putStrLn "Enter a list of numbers (sep. by commas):" >>
+    getLine >>= \input ->
+    let maybeList = getListFromString input in
+      case maybeList of
+        Just l -> return l
+        Nothing -> askUser
+
+main :: IO ()
+main = askUser >>=
+  \list -> print $ sum list

src/posts/0010-Haskell-Now/list_monad.hs

@@ -0,0 +1,14 @@
+import Control.Monad (guard)
+
+allCases = [1..10]
+
+resolve :: [(Int,Int,Int)]
+resolve = do
+              x <- allCases
+              y <- allCases
+              z <- allCases
+              guard $ 4*x + 2*y < z
+              return (x,y,z)
+
+main = do
+  print resolve

src/posts/0010-Haskell-Now/maybe_monad_1.hs

@@ -0,0 +1,30 @@
+deposit  value account = account + value
+withdraw value account = account - value
+
+eligible :: (Num a,Ord a) => a -> Bool
+eligible account =
+  let account1 = deposit 100 account in
+    if (account1 < 0)
+    then False
+    else
+      let account2 = withdraw 200 account1 in
+      if (account2 < 0)
+      then False
+      else
+        let account3 = deposit 100 account2 in
+        if (account3 < 0)
+        then False
+        else
+          let account4 = withdraw 300 account3 in
+          if (account4 < 0)
+          then False
+          else
+            let account5 = deposit 1000 account4 in
+            if (account5 < 0)
+            then False
+            else
+              True
+
+main = do
+  print $ eligible 300 -- True
+  print $ eligible 299 -- False

src/posts/0010-Haskell-Now/maybe_monad_2.hs

@@ -0,0 +1,20 @@
+deposit :: (Num a) => a -> a -> Maybe a
+deposit value account = Just (account + value)
+
+withdraw :: (Num a,Ord a) => a -> a -> Maybe a
+withdraw value account = if (account < value)
+                         then Nothing
+                         else Just (account - value)
+
+eligible :: (Num a, Ord a) => a -> Maybe Bool
+eligible account = do
+  account1 <- deposit 100 account
+  account2 <- withdraw 200 account1
+  account3 <- deposit 100 account2
+  account4 <- withdraw 300 account3
+  account5 <- deposit 1000 account4
+  Just True
+
+main = do
+  print $ eligible 300 -- Just True
+  print $ eligible 299 -- Nothing

src/posts/0010-Haskell-Now/maybe_monad_3.hs

@@ -0,0 +1,20 @@
+deposit :: (Num a) => a -> a -> Maybe a
+deposit value account = Just (account + value)
+
+withdraw :: (Num a,Ord a) => a -> a -> Maybe a
+withdraw value account = if (account < value) 
+                         then Nothing 
+                         else Just (account - value)
+
+eligible :: (Num a, Ord a) => a -> Maybe Bool
+eligible account =
+  deposit 100 account >>=
+  withdraw 200 >>=
+  deposit 100  >>=
+  withdraw 300 >>=
+  deposit 1000 >>
+  return True
+
+main = do
+  print $ eligible 300 -- Just True
+  print $ eligible 299 -- Nothing

src/posts/0010-Haskell-Now/monad_composition.hs

@@ -0,0 +1,9 @@
+import Control.Monad ((>=>))
+
+f :: Int -> [Int]
+f n = [n, n+1]
+
+g :: Int -> [String]
+g n = [show n,">"++show (n+1)]
+
+main = print $ (f >=> g) 2