On Mon, 22 Sep 2008, Andrej Bauer wrote:

> Angela Zhu wrote:
>> Hi,
>>
>> I want to define an OCaml type with constraints.
>> For example:
>>
>> type item = Item of int * float;;
>>
>> If here this float type is for price of some item, and I want to make sure
>> it is positive. In other words, if x = (xi, xf) of type item,
>> I want to enforce, xf must >= 0.
>>
>> Is there a way to define OCaml type like this?
>
> No, because Ocaml type checker always check all the types at compile
> time. It is an undecidable problem whether a given expression of type
> float evaluates to a non-negative number.

By the way, even float does not guarantee to have a number: 0./.0. 
compiles and runs happily.


Martin

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