I’m repeatedly finding myself running into decisions about sum and product
types where I’m choosing among alternatives like these:
> type FormValue valueType explanationType
> = Valid valueType
> | Invalid valueType explanationType
> | Unchecked valueType
>
> -- vs
>
> type ValidationStatus explanationType
> = Valid
> | Invalid explanationType
> | Unchecked
>
> type alias FormValue2 valueType explanationType =
> { value : valueType
> , status : ValidationStatus
> }
>
> -- vs
>
> -- Something better that makes invalid states
> -- impossible.
Too often, I realize the drawbacks of a decision way later than I’d like.
Because I was hanging around with OO people in the early 80s, I recognize this
kind of floundering. Idioms and patterns that we find obvious today were the
subject of much discussion, experimentation, and - finally - widespread
diffusion. (Though design patterns sort of took a wrong turn, and are now out
of favor, their original goal of documenting and explaining the “why” behind
certain common solutions was really powerful.)
Since FP has been around for a long time, too, I’m hoping that those idioms and
patterns have been written down, with a good effort toward explaining them
(with examples!) Where?
(What I’ve seen has been focused more on laws and provability, as opposed to
the OO emphasis on Simon’s “satisficing” [*]. Plus, the rather banal
observation that many OO patterns are unneeded or built in to FP languages led
to some strong claims (around 2000, especially) that FP systems don’t have any
design patterns - and I’m afraid the hangover from that has kept people from
writing the sort of example-based guidelines I’m looking for. Hope I’m wrong.)
[*] Satisficing is the process of looking for good-enough solutions rather than
continuing on in hopes of finding an optimal one.
https://en.wikipedia.org/wiki/Satisficing
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