[Haskell-cafe] showing a user defined type

michael rice nowgate at yahoo.com
Tue May 19 10:16:40 EDT 2009


Is there *anything* Haskell *can't* do?


--- On Mon, 5/18/09, David Menendez <dave at zednenem.com> wrote:

From: David Menendez <dave at zednenem.com>
Subject: Re: [Haskell-cafe] showing a user defined type
To: "Ryan Ingram" <ryani.spam at gmail.com>
Cc: haskell-cafe at haskell.org
Date: Monday, May 18, 2009, 10:26 PM

On Mon, May 18, 2009 at 10:02 PM, Ryan Ingram <ryani.spam at gmail.com> wrote:
> Unfortunately, you can't derive Show on Chain as defined, because it
> contains a function:

Sure you can. I just tried the following, and it compiled without complaints.

> import Text.Show.Functions
> data Chain = Link Int (Int -> Chain) deriving (Show)

The usual warnings about orphan instances apply, but the purpose of
the Text.Show.Functions module is to provide a standard Show instance
for functions so that libraries (e.g., QuickCheck) don't declare
conflicting instances.

Dave Menendez <dave at zednenem.com>
Haskell-Cafe mailing list
Haskell-Cafe at haskell.org

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