Send Beginners mailing list submissions to
[email protected]
To subscribe or unsubscribe via the World Wide Web, visit
http://www.haskell.org/mailman/listinfo/beginners
or, via email, send a message with subject or body 'help' to
[email protected]
You can reach the person managing the list at
[email protected]
When replying, please edit your Subject line so it is more specific
than "Re: Contents of Beginners digest..."
Today's Topics:
1. Re: Possible to type a function to a particular constructor?
(Jan Jakubuv)
----------------------------------------------------------------------
Message: 1
Date: Mon, 3 May 2010 12:57:34 +0100
From: Jan Jakubuv <[email protected]>
Subject: Re: [Haskell-beginners] Possible to type a function to a
particular constructor?
To: Ken Overton <[email protected]>
Cc: "[email protected]" <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Hi Ken,
You can additionally try to change your definition of lambda-terms to use
parametric types as follows.
data T var = V var | A (T var) (T var) | L var (T var)
Then you can write `freev` with the following type.
freev :: T var -> [var]
Sincerely,
jan.
On Sun, May 02, 2010 at 09:54:11AM -0400, Ken Overton wrote:
>
> Hi fellow beginners (and everyone else),
>
> As an exercise, I'm implementing a simple, untyped lambda calculus:
>
> -- a term is a variable, an application, or abstraction (lambda)
> data T = V String | A (T) (T) | L String (T)
> deriving (Eq)
>
> So I'm writing a function that returns a list of all the free variables in a
> term and descendants. I can only get it to compile with type:
>
> freev :: T -> [T]
>
> It'd be nice for the type of that function to be restricted to just variables
> like:
>
> freev :: T -> [V String] -- compile error: "Not in scope: type
> constructor or class `V'"
>
> Is there some way to express that? The error seems to suggest maybe haskell
> could do it if I'd just say it correctly. I mean, isn't "V String" a type
> constructor?
>
> Thanks,
>
> kov
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
--
Heriot-Watt University is a Scottish charity
registered under charity number SC000278.
------------------------------
_______________________________________________
Beginners mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/beginners
End of Beginners Digest, Vol 23, Issue 3
****************************************
