[Haskell-cafe] Catch-all considered harmful?

[Christopher Allen]

Addendum: it's more a smell when you aren't unconditionally ignoring
an sum type argument - it's when you're special casing _some_ behavior
for specific constructors and then not doing so for others that I
would tend to reject it on code review.

On Tue, Oct 3, 2017 at 1:02 PM, Vilem-Benjamin Liepelt <vl81 at kent.ac.uk> wrote:
> 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.)
>>
>>
>
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-- 
Chris Allen
Currently working on http://haskellbook.com