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@OliverJAsh
Last active September 4, 2023 15:31
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`Option` vs non-`Option`

Option vs non-Option

Option<T> non-Option (T | undefined)
accessing property userOption.map(user => user.age) userNullish?.age
calling a method userOption.map(user => user.fn()) userNullish?.fn()
providing fallback ageOption.getOrElse(0) ageNullish ?? 0
filter ageOption.filter(checkIsOddNumber) ageNullish !== undefined && checkIsOddNumber(ageNullish) ? ageNullish : undefined
map ageOption.map(add1) ageNullish !== undefined ? add1(ageNullish) : undefined
flat map / chain ageOption.flatMap(add1) ageNullish !== undefined ? add1(ageNullish) : undefined
check for existence with predicate ageOption.exists(checkIsOddNumber) ageNullish !== undefined ? checkIsOddNumber(ageNullish) : false
check for existence with method nameOption.exists(name => name.startsWith('bob')) nameNullish?.startsWith('bob') ?? false
nesting Option<Option<T>> impossible
sequencing sequence(fa, fb) fa !== undefined ? fb !== undefined ? [fa, fb] : undefined : undefined
mapping multiple sequence(fa, fb).map(add) fa !== undefined ? fb !== undefined ? add([fa, fb]) : undefined : undefined

Comparison of advanced example

const age1 = userOption
  .flatMap(user => user.age)
  .map(plus1)
  .filter(checkIsOddNumber)
  .getOrElse(0);

const age2 =
  (user?.age !== undefined
    ? (() => {
        const agePlus1 = plus1(user.age);
        return checkIsOddNumber(agePlus1) ? agePlus1 : undefined;
      })()
    : undefined) ?? 0;
@baetheus
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I see, for the case where you are mapping with a function f where f<T>(ta: Nilable<T>): Nilable<T> you lose information during composition. Effectively this is the case where undefined is meaningful as a value.

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