Why pattern matching does not throw exception in Maybe monad.

Question

Hi all. My question is simple. Why wrong pattern matching does not throw exception in Maybe monad. For clarity :

data Task = HTTPTask {
 getParams   ::  [B.ByteString],
 postParams  ::  [B.ByteString],
 rawPostData ::  B.ByteString 
}  deriving (Show)

tryConstuctHTTPTask :: B.ByteString -> Maybe Task
tryConstuctHTTPTask str = do
 case decode str of
    Left _  -> fail ""
    Right (Object trie) -> do
        Object getP    <- DT.lookup (pack "getParams")   trie
        Object postP   <- DT.lookup (pack "postParams")  trie
        String rawData <- DT.lookup (pack "rawPostData") trie
        return $ HTTPTask [] [] rawData

Look at tryConstuctHTTPTask function. I think that when the pattern does not match (for example "Object getP") we must get something like "Prelude.Exception", instead i get the "Nothing". I like this behavior but i am not understand why.

Thanks.

Answer 1

Doing pattern <- expression in a do-block, will call fail when the pattern does not match. So it is equivalent to doing

expression >>= \x ->
case x of
  pattern -> ...
  _ -> fail

Since fail is defined as Nothing in the Maybe monad, you get Nothing for failed pattern matches using <-.