102 lines
2.2 KiB
Markdown
102 lines
2.2 KiB
Markdown
# Reductions
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In Tomo, reductions are a way to express the idea of folding or reducing a
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collection of values down to a single value. Reductions use a parenthesized
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infix operator followed by a colon, followed by a collection:
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```tomo
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nums := [10, 20, 30]
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sum := (+: nums)
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>> sum
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= 60 : Int?
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```
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Reductions return an optional value which will be a null value if the thing
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being iterated over has no values. In such cases, the reduction is undefined.
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As with all optionals, you can use either the postfix `!` operator to perform
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a runtime check and error if there's a null value, or you can use `or` to
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provide a fallback value:
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```tomo
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nums := [:Int]
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sum := (+: nums)
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>> sum
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= NULL : Int?
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>> sum or 0
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= 0
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>> nums = [10, 20]
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>> (+: nums)!
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= 30
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```
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Reductions can be used as an alternative to generic functions like `sum()`,
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`product()`, `any()`, and `all()` in Python, or higher-order functions like
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`foldl` and `foldr` in functional programming:
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```tomo
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# Sum:
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>> (+: [10, 20, 30])!
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= 60
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# Product:
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>> (*: [2, 3, 4])!
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= 24
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# Any:
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>> (or: [no, yes, no])!
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= yes
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# All:
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>> (and: [no, yes, no])!
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= no
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```
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## Minimum and Maximum
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Reductions are _especially_ useful for finding the minimum or maximum values in
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a collection using the `_min_` and `_max_` infix operators.
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```tomo
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# Get the maximum value:
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>> (_max_: [10, 30, 20])!
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= 30
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# Get the minimum value:
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>> (_min_: [10, 30, 20])!
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= 10
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```
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The `_min_` and `_max_` operators also support field and method call suffixes,
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which makes it very easy to compute the argmin/argmax (or keyed
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minimum/maximum) of a collection. This is when you want to get the minimum or
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maximum value _according to some feature_.
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```tomo
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# Get the longest text:
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>> (_max_.length: ["z", "aaaaa", "mmm"])!
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= "aaaaa"
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# Get the number with the biggest absolute value:
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>> (_max_:abs(): [1, -2, 3, -4])!
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= -4
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```
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## Comprehensions
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Reductions work not only with iterable values (arrays, sets, integers, etc.),
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but also with comprehensions. You can use comprehensions to perform reductions
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while filtering out values or while applying a transformation:
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```tomo
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# Sum the lengths of these texts:
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>> (+: t.length for t in ["a", "bc", "def"])!
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= 6
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# Sum the primes between 1-100:
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>> (+: i for i in 100 if i:is_prime())!
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= 1060
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```
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