Re: [Haskell-cafe] Error Monad and strings
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 it was written that way. Data.Map.lookup
used to call "fail" when it could not find a key, but that got changed.
If you don't want to catch these errors in your monad, you can write
your own monad (or monad transformer). For example:
newtype ErrorCode = ErrorCode Int deriving Show
newtype ErrorCodeT m a = ErrorCodeT { runErrorCodeT :: m (Either
ErrorCode a) }
instance Monad m => Monad (ErrorCodeT m) where
return = ErrorCodeT . return . Right
a >>= b = ErrorCodeT $ do
m <- runErrorCodeT a
case m of
Left err -> return $ Left err
Right x -> runErrorCodeT $ b x
fail = ErrorCodeT . fail
failWithCode :: Monad m => Int -> ErrorCodeT m a
failWithCode = ErrorCodeT . return . Left . ErrorCode
There's probabaly a library somewhere which does this already.
On 2010 July 27, at 16:08, Gerald Gutierrez wrote:
I see. So strings must be supported in the case of a bug which
cannot be caught at compile time? In other words, if I get an error
with a string, I'm pretty much guaranteed it is a bug, i.e. a
pattern match error as the "fail" documentation says.
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Re: [Haskell-cafe] Error Monad and strings
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 } :: Either String Int
Left "Pattern match failure in do expression at :1:5-8"
Note that in the "Either String" monad, "failStr" is equal to "Left".
On 2010 July 27, at 15:32, 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 documentation, so why the need to always
support strings via noMsg/strMsg ? What uses these? And if in my
code, I will never throw an error with a string, am I supposed to
implement these functions and then ignore them?
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Re: [Haskell-cafe] Why is it so different between 6.12.1 and 6.10.4_1 ?
I think Miljenovic was asking about this (I removed explicit braces):
fac n = let f = foldr (*) 1 [1..n] in f
Which is strictly equivalent to:
fac n = foldr (*) 1 [1..n]
Translated into C, this is kind of like doing this:
int add(int x, int y)
{
int sum = x + y;
return sum;
}
instead of this:
int add(int x, int y)
{
return x + y;
}
I find it very cumbersome (though not *difficult*) and painful to use
a C style of programming with Haskell, so I am not sure what you mean
when you ask why Haskell supports C style. Are you talking about
mutable state, syntax, or something else?
--Dietrich
On 2010 March 27, at 4:28, zaxis wrote:
Of course, you are wrong ! C is VERY important for almost every
programmer
in the world! Why cannot C programmer use haskell ? And Why does
haskell
support C code style ?
Ivan Miljenovic wrote:
zaxis writes:
Why do you bother with the interior definition of f in there?
Because i want to try a C code style not layout style without `do`
syntax
sugar .
Haskell /= C, so stop trying to code as if it is. If you like C so
much, then use C.
--
Ivan Lazar Miljenovic
ivan.miljeno...@gmail.com
IvanMiljenovic.wordpress.com
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fac n = let { f = foldr (*) 1 [1..n] } in f
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[Haskell-cafe] Problem with Haskell Platform website
There's a problem with the Haskell Platform website. I'm posting the message here because I gave up looking for contact information for the site, maybe someone here can forward the message or tell me who to forward it to? The specific page is http://hackage.haskell.org/platform/mac.html Being one of the few who still use the PPC architecture, I expect to have to build my own packages from source. Imagine my surprise when the Haskell platform uses the "Universal Binary" logo on the Mac download. Nowhere on the Mac download page does it specify an architecture. However, the binary is i386 only and not PPC. So I just wasted time downloading something I can't install on my computer. This page shows the "Universal Binary" logo: http://hackage.haskell.org/platform/ Here is Apple's "Universal Binary" page, which describes what the logo means: http://www.apple.com/universal/ It would be nice to get rid of the "Universal Binary" logo and to mark the packages as i386. --Dietrich Epp ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
