Re: [Haskell-cafe] Either e Monad

[Deokhwan Kim]

Albert Lai wrote:
 Deokhwan Kim [EMAIL PROTECTED] writes:

 Where is the Monad instance declaration of Either e?

 It is in Control.Monad.Error as well.  Strange: the doc doesn't state it.

Thanks a lot, Albert! I found the declaration in 
libraries/mtl/Control/Monad/Error.hs of the ghc source distribution:


  instance (Error e) = Monad (Either e) where
  return= Right
  Left  l = _ = Left l
  Right r = k = k r
  fail msg  = Left (strMsg msg)

--
Deokhwan Kim
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[Haskell-cafe] Either e Monad

[Deokhwan Kim]

Dear all,

Where is the Monad instance declaration of Either e?

From the description of Control.Monad.Error, I deduce that Either e is
an instance of Monad.


http://haskell.org/ghc/docs/latest/html/libraries/mtl/Control-Monad-Error.html

  class Monad m = MonadError e m | m - e where ...

  Error e = MonadError e (Either e)

But, I cannot find the Monad instance declaration of Either anywhere.

Thanks.

-- 
Deokhwan Kim
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[Haskell-cafe] Monad laws

[Deokhwan Kim]

What is the practical meaning of monad laws?

(M, return, =) is not qualified as a category-theoretical monad, if
the following laws are not satisfied:

  1. (return x) = f == f x
  2. m = return == m
  3. (m = f) = g == m  (\x - f x = g)

But what practical problems can unsatisfying them cause? In other words,
I wonder if declaring a instance of the Monad class but not checking it
for monad laws may cause any problems, except for not being qualified as
a theoretical monad?

Cheers,

-- 
Deokhwan Kim
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[Haskell-cafe] The values of infinite lists

[Deokhwan Kim]

Are the values of infinite lists _|_ (bottom)?

In section 1.3, the Haskell 98 report said as follows:

  Errors in Haskell are semantically equivalent to _|_. Technically,
  they are not distinguishable from nontermination, so the language
  includes no mechanism for detecting or acting upon errors.

Therefore, the value of the following infinity is _|_. Right?

  data Nat = Zero | Succ Nat

  infinity = Succ infinity

What about infinite lists? For example, is the value of [1 ..] also _|_?

Thanks.
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