On Tue, Jul 27, 2010 at 04:08:39PM -0700, Gerald Gutierrez wrote:
Reading the Control.Monad.Error documentation, I see that the Error class
has noMsg and strMsg as its only two functions.
Now, I understand that you can define your own Error instances such as in
example 1 of the
I've always thought that being able to write:
catMaybes :: [Maybe a] - [a]
catMaybes xs = [ x | Just x - xs ]
is really cool, which relies on:
fail _ = []
being in the Monad instance for List.
Really? I thought that's just a feature of list comprehensions. List
comps are not monads,
Reading the Control.Monad.Error documentation, I see that the Error class
has noMsg and strMsg as its only two functions.
Now, I understand that you can define your own Error instances such as in
example 1 of the documentation, so why the need to always support strings
via noMsg/strMsg ? What
It is for the very annoying reason that in order for Error to be a monad
it has to implement the fail method, which means it has to know how to
turn an arbitrary string into a value of your error type.
Cheers,
Greg
On 07/27/10 15:32, Gerald Gutierrez wrote:
Reading the Control.Monad.Error
The strMsg method is used to implement the fail method in the
resulting method, and calls to fail might be inserted into your code
even if you don't explicitly call it. An example in GHCi:
Prelude :m + Control.Monad.Error
Prelude Control.Monad.Error do { Just x - return Nothing ; return
x
On Tue, Jul 27, 2010 at 3:57 PM, Dietrich Epp d...@zdome.net wrote:
The strMsg method is used to implement the fail method in the resulting
method, and calls to fail might be inserted into your code even if you
don't explicitly call it. An example in GHCi:
Prelude :m + Control.Monad.Error
I'll say yes, a pattern match failure is a bug. This is one of the
great debates in the language: whether all pattern matching code
should be guaranteed complete at compile time or not. However, any
function you call which returns a result in your monad could
theoretically call fail if
On Tue, Jul 27, 2010 at 6:29 PM, Dietrich Epp d...@zdome.net wrote:
I'll say yes, a pattern match failure is a bug. This is one of the great
debates in the language: whether all pattern matching code should be
guaranteed complete at compile time or not. However, any function you call
which
