lab08: impl

lab08/monadLab.lhs

@@ -32,12 +32,19 @@ but without the do syntax.
 Implement Times and Divide.  Try the Times case without monads (as we did with
 the Plus case).
 
-> applyOp Times mi mj = error "TBD"
+> applyOp Times mi mj = 
+>   case mi of
+>     Nothing -> Nothing
+>     Just i ->
+>       case mj of
+>         Nothing -> Nothing
+>         Just j -> Just $ i * j
 
 For the Divide case, use bind (>>=) as we did for Minus.
 On an attempt to divide by 0, return Nothing as the answer.
 
-> applyOp Divide mi mj = error "TBD"
+> applyOp Divide mi mj =
+>   mi >>= (\i -> mj >>= (\j -> if j == 0 then Nothing else Just $ i `div` j))
 
 The following test cases will help you verify your changes.
 
@@ -53,9 +60,20 @@ The Plus case is done for you once again.  Be sure to check for zero with Divide
 >   i <- mi
 >   j <- mj
 >   return $ i + j
-> applyOp' Minus mi mj = error "TBD"
+> applyOp' Minus mi mj = do
-> applyOp' Times mi mj = error "TBD"
+>   i <- mi
-> applyOp' Divide mi mj  = error "TBD"
+>   j <- mj
+>   return $ i - j
+> applyOp' Times mi mj = do
+>   i <- mi
+>   j <- mj
+>   return $ i * j
+> applyOp' Divide mi mj  = do
+>   i <- mi
+>   j <- mj
+>   if j == 0
+>     then Nothing
+>     else return $ i `div` j
 
 More test cases:
 
@@ -75,6 +93,28 @@ Experiment with these functions and the >>= syntax.
 Here is one example:
 
 > testIncDec = Just 7 >>= mincr >>= mincr >>= mincr >>= mdecr
+> testIncDec' = do
+>   i <- Just 7
+>   i <- mincr i
+>   i <- mincr i
+>   i <- mincr i
+>   i <- mdecr i
+>   return i
 
 Does bind seem more natural in this case than using do?  Why or why not?
 
+The bind behaves sort of like an upgraded pipe `|` from shell, 
+where the output of one function is piped into the next function,
+but with the added benefit of error handling
+
+The do syntax is a bit more explicit in this case, as it requires unwrapping the value,
+performing the operation, and then wrapping the value back up
+
+> main :: IO ()
+> main = do
+>   print testapp1
+>   print testapp2
+>   print testapp1'
+>   print testapp2'
+>   print testIncDec
+>   print testIncDec'