[Haskell-cafe] Catch-all considered harmful?

Vilem-Benjamin Liepelt vl81 at kent.ac.uk
Tue Oct 3 17:59:15 UTC 2017

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.


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).


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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