On Wed, Jun 8, 2011 at 9:25 PM, Tyson Whitehead
Hi All,
I've been working my way through Yesod and noticed an irregularity.
The GGHanlder monad stack (Yesod/Handler.hs)
type GHInner s m monad a = ReaderT (HandlerData s m) ( ErrorT HandlerContents ( WriterT (Endo [Header]) ( StateT GHState monad ) ) ) a
has a WriterT member based on "Endo [Header]", which is locally defined as
type Endo a = a -> a
Therefore, unless I'm missing something, the Writer T Monoid instance must be
Monoid b => Monoid (a -> b) Monoid [a]
(i.e., an environment to list construct). Assuming the results a finally extracted by applying [] (an empty environment), this would mean
addHeader = GHandler . lift . lift . tell . (:)
degrades to (++), giving non-constant time concatenation.
I'm thinking what was really meant was to use the Endo Monoid instance, thereby gaining the constant-time concatenation via function composition.
Cheers! -Tyson
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Hi Tyson, Awesome catch, that's most definitely what was intended. I'll apply a patch. Thanks, Michael