For comprehensions

A list comprehension maps each element of a list:

[ for s in input.services : s.name ]

Add a when clause to filter elements:

[ for s in input.services : s.name when s.public ]

A map comprehension uses key => value:

{ for s in input.services : s.name => s.port }

For maps, bind both key and value:

{ for name, port in input.ports : name => port }

Use ... after the value to group repeated keys into lists:

{ for s in input.services : s.region => s.name... }

Comprehensions can nest:

[
  for r in input.regions :
  { region: r, names: [ for s in input.services : s.name when s.region == r ] }
]

Filters also narrow optional values. In this example, u.port is known to be an integer in the value expression:

[ for u in input.upstreams : u.port when u.port != null ]

A splat is a shortened form of comprehension. The following are equivalent:

subnets-a: resource.subnets[*].id
subnets-b: [ for s in resource.subnets : s.id ]