[ruby-core:119469] [Ruby master Feature#20786] Flow chaining with "then" keyword

6 Oct 2024

      Issue #20786 has been reported by lpogic (Łukasz Pomietło).

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Feature #20786: Flow chaining with "then" keyword
https://bugs.ruby-lang.org/issues/20786

* Author: lpogic (Łukasz Pomietło)
* Status: Open
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Hi,
I would like to propose using the *"then"* keyword to create chained flows.

### Background:
Original idea: https://bugs.ruby-lang.org/issues/20770#change-110056
However, the concept has changed slightly since then. I now assume that rhs is only calculated when lhs is true, which seems more compatible with the current usage of *"then"* keyword.
I would also like to move the discussion here as it may cause unnecessary chaos there.

### Basics:
In the canonical version, *"then"* is used in a *"begin..end"* block to split it into steps.
When the result of the last expression before *"then"* is not *"false"* or *"nil"*, it is passed as an argument to the next step. Otherwise, all subsequent steps are skipped.
The result of the *"begin..end"* block is the result of the last expression, or *"false"*/*"nil"* if any step was skipped.

### Example:
``` ruby
# original code source: https://github.com/ruby/ruby/blob/a6383fbe1628bdaa9ec6a30b3baa60dd7430b461/l...
def include?(other)
  other = coerce_other(other)
  return false unless other.family == family
  begin_addr <= other.begin_addr && end_addr >= other.end_addr
end

# with chained flow
def include?(other)
  begin
    coerce_other(other)
  then => coerced  # then => name   is my proposition of argument naming syntax
    return false unless coerced.family == family
    coerced
  then => it
    begin_addr <= it.begin_addr && end_addr >= it.end_addr
  end
end
```

Please treat the canonical form as a starting point. As further improvements I would see:
- implicit block argument name
- implicit begin
- beginless *"then"*, when lhs is just expression

### Example with futher improvements:
``` ruby
def include?(other)
  coerce_other(other)
then 
  it.family == family ? it : false
then
  begin_addr <= it.begin_addr && end_addr >= it.end_addr
end

# or
def include?(other)
  coerce_other other then
    return false unless it.family == family
    begin_addr <= it.begin_addr && end_addr >= it.end_addr
  end
end
```

### *"then"* keyword as the pipeline operator:
Under certain conditions, "then" could offer similar capabilities to the pipeline operator of functional languages:
``` ruby
# assuming that "foo" and "bar" never return "false" or "nil"
# instead of writing this:
baz("string", bar(foo(), 2), 5)
# you could write this:
begin foo() 
then bar(it, 2) 
then baz("string", it, 5) 
end
```

-- 
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