[Haskell-cafe] Catch-all considered harmful?

[Vilem-Benjamin Liepelt]

PS: Some people have already commented on this here: https://gist.github.com/vimuel/ee9b054b42bbc2ed06992a323b7dfbd8

> On 2017-10-03, at 18:59, Vilem-Benjamin Liepelt <vl81 at kent.ac.uk> 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.)
> 
>