Haskell newtype syntax
haskell, newtype
Solution
Because `runWriter` is a record field accessor on `Writer`. It's actually almost equivalent to
runWriter (Writer x) = x
Haskell just has records to give
- More convient syntax since this sort of accessor code is quite common
- The ability for functional updates
- A few other extensions
eg
someWriter{runWriter = (new, values)} -- Returns a new Writer.
If it helps, think of it as something like a "functional getter" in the roughest sense. This might not seem terribly important with 1 field, you can always pattern match, but when you have 5 fields, records + functional updates are super helpful. See LYAH for a more in depth explanation.
Problem
Please help me to understand the following definition: ``` newtype Writer w a = Writer { runWriter :: (a,w) } instance (Monoid w) => Monad (Writer w) where return a = Writer (a,mempty) (Writer (a,w)) >>= f = let (a',w') = runWriter $ f a in Writer (a',w `mappend` w') ``` Why is runWriter is declared as ``` runWriter :: (a,w) ``` when its actual type is: ``` runWriter :: Writer w a -> (a, w) ``` Once I tried with ghci I realized this must be some implicit argument since type "a" has to be determined, but what is exactly happening here?