I just couldn't figure out what (fun _ -> postcode) does. But when I factored out the composed modifier, I realized that (fun _ -> postcode) was the argument that is passed to Address.modifyPostcode:
The (fun _ -> postCode) does look a bit funny yeah. It’s just a lambda that ignores its input and always returns the value of the postCode argument that was passed in to the outer function. So it’s just a transformer that doesn’t depend on the current value. This is all setters are, they just overwrite what is currently stored with a completely new value.
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Sorry, I tried to upload the screenshot again, this time on desktop, but it didn't appear.
This is the function I have been struggling with until about five minutes ago:
let setCreditCardPostcode (postcode: Postcode) =
(Address.modifyPostcode
>> CreditCard.modifyAddress
>> User.modifyCreditCard)
(fun _ -> postcode)
I just couldn't figure out what (fun _ -> postcode) does. But when I factored out the composed modifier, I realized that (fun _ -> postcode) was the argument that is passed to Address.modifyPostcode:
// (Postcode -> Postcode) -> (User -> User)
let modifyCreditCardPostcode =
Address.modifyPostcode
>> CreditCard.modifyAddress
>> User.modifyCreditCard
// Postcode -> (User -> User)
let setCreditCartPostcode (postcode: Postcode) =
modifyCreditCardPostcode (fun _ -> postcode)
Actually, you explained this in your post, but somehow (fun _ -> postcode) just looked alien to me ;-)
Now that I understand this function, the rest of the post will make much more sense for me.
Thanks again for taking on such a difficult to explain topic and make it accessible!
Awesome yeah you’ve got it! 🙌
The
(fun _ -> postCode)does look a bit funny yeah. It’s just a lambda that ignores its input and always returns the value of thepostCodeargument that was passed in to the outer function. So it’s just a transformer that doesn’t depend on the current value. This is all setters are, they just overwrite what is currently stored with a completely new value.