220 2147 <CANh-dXnF6yCyYWwwA0DGbot5EBufUkQqoz8sy-hRqHN-JmGeiA@mail.gmail.com> article
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From: Jeffrey Yasskin <jyasskin@googlers.com>
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Subject: Re: optional<T>, one more value or two states
Date: Thu, 24 Jan 2013 10:15:58 -0800
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On Thu, Jan 24, 2013 at 10:13 AM, Zhihao Yuan <lichray@gmail.com> wrote:
> On Thu, Jan 24, 2013 at 12:01 PM, Jeffrey Yasskin <jyasskin@googlers.com> wrote:
>> optional<T> has good reasons to make its
>> conversion implicit, and if convert() is adopted, those reasons
>
> My reason to support explicit conversion is a logical reason.
> Think of type T with possible values { a, b, c }, and U with
> possible values { a, b, c, x }, we can say T is-a U, since T can
> be used everywhere U can be used.  And in such a case,
> an implicit conversion from T to U is logically.
>
> But optional<T> should be different.  In type theory, it's
> T' build upon T, with possible values { a', b', c', None }.  An
> implicit conversion should not be allowed here.  And no
> existing functional programming language has an logic
> such that to use T instead of `Some T`.

Which functional languages have user-defined implicit conversions at all?

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