On Sat, Dec 08, 2007 at 02:59:16PM -0200, Felipe Lessa wrote:
Hello!
I see from http://www.haskell.org/haskellwiki/Monads_as_computation#Do_notation that
do { v <- x ; <stmts> } = x >>= \v -> do { <stmts> }
However, look at this GHCi session:
Prelude> let return' = return :: a -> Maybe a Prelude> do {1 <- return 1; return' "ok"} Just "ok" Prelude> return 1 >>= \1 -> return' "ok" Just "ok" Prelude> do {1 <- return 3; return' "ok"} Nothing Prelude> return 3 >>= \1 -> return' "ok" *** Exception: <interactive>:1:13-30: Non-exhaustive patterns in lambda What seems confusing to you?
\1 -> foo is the same as \x -> case x of {1 -> foo;} When this function is evaluated with parameter different from 1, Haskell fails to find matching pattern for x and exception occurs.