Re: RFC: Add HasCallStack constraint to partial Data.List functions.

The sad part is seemingly it discards having an informative stack trace? On Wed, Jun 9, 2021 at 10:16 AM Simon Peyton Jones via Libraries < libraries@haskell.org> wrote:

| If we indeed had something like | | head :: Partial => [a] -> a | | that would be both informative and fairly straightforward to explain to | students, for example. (Even if it is not clear to me that a type class | really is the right way to express partiality of functions: I always thought | information about partiality ought to be tied to the function arrow.)

OK -- that sounds promising. It's what Richard suggested earlier, and sounds pretty good to me.

Simon

| -----Original Message----- | From: Henrik.Nilsson@nottingham.ac.uk | Sent: 09 June 2021 14:45 | To: Simon Peyton Jones ; Henrik Nilsson | ; libraries@haskell.org | Subject: Re: RFC: Add HasCallStack constraint to partial Data.List | functions. | | > I'm not sure I really agree with that. There is a rich literature on > | effect systems, which decorate types with information about what > effects | the function has: exceptions, divergence, IO, and the like. | > So type like | > head :: Partial => [a] -> a | > where 'Partial =>' expresses the fact that calling this function > might | lead to a call of 'error' doesn't seem inherently something > that doesn't | belong in a type system. | | I, of course, agree that partiality is an effect. And I have no issues with | effects being reflected in the type system. | We do that all the time with e.g. monads. | | If we indeed had something like | | head :: Partial => [a] -> a | | that would be both informative and fairly straightforward to explain to | students, for example. (Even if it is not clear to me that a type class | really is the right way to express partiality of functions: I always thought | information about partiality ought to be tied to the function arrow.) | | My point is that "HasCallStack" strongly suggest a specific approach to | monitor the behaviour of a function in case it goes wrong. | | To me, at least, that is very operational. | | And I would struggle to explain | | head :: HasCallStack => [a] -> a | | beyond saying "it's just something that sometimes will help you with | debugging", and deeply hoping no clever student would ask about the lack of | similar annotations for other partial functions. | | Best, | | /Henrik | | | | This message and any attachment are intended solely for the addressee and | may contain confidential information. If you have received this message in | error, please contact the sender and delete the email and attachment. | | Any views or opinions expressed by the author of this email do not | necessarily reflect the views of the University of Nottingham. Email | communications with the University of Nottingham may be monitored where | permitted by law. | | |

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries