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

On Fri, Jun 07, 2013 at 04:05:09PM -0400, Joe Q wrote:

The phantom parameter solves the same problem as scoped type variables. Granted, if you find yourself in that kind of polymorphic soup you have deeper problems...

I don't understand this. Scoped type variables are used when you want to use a type variable from the top level within the body of a function. If you use "op" and specify a particular constructor then you don't have a variable but a concrete instance of a type. But maybe I'm missing some more powerful way this can be used ... Tom

On Jun 7, 2013 2:53 PM, "Tom Ellis" wrote:

...

...

op :: Newtype< http://hackage.haskell.org/packages/archive/newtype/0.2/doc/html/Control-New...

n o => (o -> n) -> n -> oSource< http://hackage.haskell.org/packages/archive/newtype/0.2/doc/html/src/Control...

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

On Fri, Jun 07, 2013 at 07:08:19AM -0700, David Banas wrote: 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

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe