[plt-scheme] optional arguments in function contracts

That contract is one that requires the second argument (none/c used
like that means "you must supply a value, but I won't accept it"
roughly, so isn't what you want).

This is the relevant section of the guide that explains optional arguments:

http://docs.plt-scheme.org/guide/contracts-general-functions.html#(part._contracts-optional)

and this is the relevant section of the reference manual (for all the details):

http://docs.plt-scheme.org/reference/Function_Contracts.html

Robby

On Fri, Jun 27, 2008 at 4:52 AM, Stephen De Gabrielle
<spdegabrielle at gmail.com> wrote:
> HI,
>
> I'm trying to do a contract with an optional argument, but I'm stuck
> (and can't find an example in the doc's, tests or 'plt-full' code)
> my provide/contract is;
> ;; write-email accepts an email and optionally an output-port (2 or one args)
> (provide/contract
>  (write-email (email? (or/c none/c output-port?) . -> . any))
>  )
>
>
> and my function is
> ;; write-email : message [output-port] -> void
> (define write-email
>  (lambda (message (out-port (current-output-port)))
>    (printf "~V:~V~N" message out-port)
>    )
>  )
>
> but calling it I get the error; procedure write-email: expects 2
> arguments, given 1:
>
> Forgive me if this is a dumb question but I feel I'm missing something
> fundamental about how I'm specifying the contract?
>
>
> Cheers,
>
> Stephen
>
> --
> Stephen De Gabrielle
> s.degabrielle at cs.ucl.ac.uk
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