On Wed, Jun 06, 2007 at 01:39:32PM +0400, Dmitri O.Kondratiev wrote:
Monad class contains declaration
*fail* :: String -> m a
and provides default implementation for 'fail' as:
fail s = error s
On the other hand Prelude defines: * error* :: String -> a
which stops execution and displays an error message.
Questions: 1) What value and type 'error' actually returns in: error "some message" ? 'a' here is a type variable, therefore 'error' is a polymorphic function which can return value of any type.
2) How declaration String -> m a matches with String -> a ? as I said above, 'String->a' means that the return value can have any type. 'String-> m a' means that return value may be of type 'm a' where 'a' can be any type.
3) In Maybe monad: fail = Nothing 'Nothing' is constructor which returns value of type 'Maybe a' where 'a' is any type.
When and how 'fail' is used in Maybe monad? Values of type 'Maybe a' can be either 'Just x' where x is value of type 'a' or 'Nothing'. The Maybe monad is used to represent computations which may fail, 'Just x' represents successful computation yielding value x, and 'Nothing' represents failing computation.
Thanks!
-- Dmitri O. Kondratiev dokondr@gmail.com http://www.geocities.com/dkondr
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