Haskell newtype syntax -

please me understand following definition:

newtype writer w = writer { runwriter :: (a,w) }   instance (monoid w) => monad (writer w)      return             = writer (a,mempty)      (writer (a,w)) >>= f = let (a',w') = runwriter $ f in writer (a',w `mappend` w') 

why runwriter declared as

runwriter :: (a,w) 

when actual type is:

runwriter :: writer w -> (a, w) 

once tried ghci realized must implicit argument since type "a" has determined, happening here?

because runwriter record field accessor on writer. it's equivalent to

runwriter (writer x) = x 

haskell has records give

1. more convient syntax since sort of accessor code quite common
2. the ability functional updates
3. a few other extensions

eg

somewriter{runwriter = (new, values)} -- returns new writer. 

if helps, think of "functional getter" in roughest sense. might not seem terribly important 1 field, can pattern match, when have 5 fields, records + functional updates super helpful. see lyah more in depth explanation.