[Haskell-cafe] Question about Newtype "op()" function arguments.

[Tom Ellis]

On Fri, Jun 07, 2013 at 07:08:19AM -0700, David Banas wrote:
> op :: Newtype<http://hackage.haskell.org/packages/archive/newtype/0.2/doc/html/Control-Newtype.html#t:Newtype>
> n
> o => (o -> n) -> n ->
> oSource<http://hackage.haskell.org/packages/archive/newtype/0.2/doc/html/src/Control-Newtype.html#op>
> 
> This function serves two purposes:
> 
>    1. Giving you the unpack of a newtype without you needing to remember
>    the name.
>    2. Showing that the first parameter is *completely ignored* on the value
>    level, meaning the only reason you pass in the constructor is to provide
>    type information. Typeclasses sure are neat.
> 
> As point #2, above, emphasizes, the only purpose for the first argument to
> the function (i.e. - the constructor "(o -> n)") is to specify the type of
> 'n'. However, being a *newtype*, 'n' can have only one constructor. So, why
> is it necessary to pass in the constructor to this function, when we're
> already passing in 'n'?

I am puzzled by this too.

What does "op" stand for?  I hypothesis "opposite" in the sense of inverse,
since as Roman points out "op Constructor :: n -> o" is the inverse of
"Constructor :: o -> n".

But I admit I do not see how this provides value over "unpack" itself.

Tom