[Haskell-cafe] Catch-all considered harmful?

Christopher Allen cma at bitemyapp.com
Tue Oct 3 18:04:58 UTC 2017

We made it a policy at a previous company using Haskell to not use
catch-all patterns whenever possible because it meant adding a new
value to a sum type could mean silent problems. We had one bad
experience with that, did the five-whys thing, never did it again.

This mostly applied to the data types we made to represent domain
specific information. Less true for stuff like `Int`, naturally.

On Tue, Oct 3, 2017 at 12:59 PM, 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.)
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Chris Allen
Currently working on http://haskellbook.com

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