[Haskell-cafe] Catch-all considered harmful?

[Li-yao Xia]

There was a discussion about that here some time ago:

https://mail.haskell.org/pipermail/haskell-cafe/2017-January/126157.html

Li-yao

On 10/03/2017 01:59 PM, Vilem-Benjamin Liepelt wrote:
> Catch-all considered harmful?
> =============================
>
> I have been thinking about a potential source of bugs from catch-all pattern matches on sum types and would like to know your thoughts.
>
>
> Motivation
> ----------
>
> Totality is usually a desirable property of a function and the catch-all can conveniently buy us totality. But at what price?
>
> I have been indoctrinated that rigour goes above convenience (think along the lines of: "Once we indulge in the impurities of I/O, there is no redemption.")
>
> I would like to evaluate the trade-offs between convenience for the programmer and a potential source of bugs.
>
> My questions to the community—
>
> 1. Are there real world examples of bugs caused by catch-alls?
> 2. Do you think that a language extension that disallows catch-alls (and annotations to opt back in at pattern match sites or type declaration) could be useful for certain code bases?
> 3. If this is a potential problem, then can you think of any better solutions a compiler could provide (i.e. that don't rely on an IDE / structured editing) other than disallowing catch-alls?
>
> Feel free to chip in with your 2p (or 2¢), but please only if you have any concrete experience (or compelling theoretical evidence).
>
>
> Example
> -------
>
> Consider the sum type:
>
>      data Answer = No | Yes
>
> and the function:
>
>      foo : Answer -> String
>      foo Yes = "Woo-hoo!"
>      foo _   = "Bother."
>
> Say we need to extend our sum type:
>
>      data Answer = No | Perhaps | Yes
>
> However, we forget to handle the new case appropriately in `foo`. The compiler is happy, but at runtime `foo Perhaps` would evaluate to `"Bother."`—with potentially catastrophic consequences.
>
> (Please imagine this happening in a large codebase with several contributors, no single one of whom knows the entire codebase.)
>
>
>
>
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.