fsprojects / fsprojects/FSharpPlus
Using setl and view for the same optic in one function gives error "The type Data.Identity does not match the type Data.Const"
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- F#
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Description
I'm trying to use optics to set a record field together with the record's UpdatedAt: DateTimeOffset field only if the field value has changed.
I have created the following generic function, which doesn't compile:
let inline private setField valueOptic value updatedAtOptic now source =
let oldVal = view valueOptic source
if value = oldVal then source
else
setl valueOptic value source
|> setl updatedAtOptic now
What am I doing wrong?
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Research direction
Start with the inline setField function and reproduce the view/setl combination in a minimal F# example. Read how the optics API types view and setl, then verify that the function compiles and conditionally updates both the selected field and UpdatedAt.
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Assessment
- Domain
- tooling
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100