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comprehension-term-assignment

Summary: Assignment can be moved to comprehension term

Category: Style

Avoid

package policy

import rego.v1

names := [name |
    some user in input.users
    name := user.name # redundant assignment
]

Prefer

package policy

import rego.v1

names := [user.name |
    some user in input.users
]

# which in this case can be made a one-liner
names := [user.name | some user in input.users]

Rationale​

Adding an intermediate assignment in a comprehension body to a variable used as the comprehension term (i.e. the value to the left side of | in a comprehension) is redundant, as the value can be used directly as the comprehension term. Making code as compact as possible should never be a goal in itself, but the same is true for making code needlessly verbose. And in cases like names := [user.name | some user in input.users], adding an intermediate assignment does nothing to improve readability.

Exceptions​

This rule will only flag simple assignments where the value could be moved directly into the comprehension term. More complex assignments involving dynamic references or function calls, will not be considered as violations.

Example:

first_names := [first_name |
    some user in input.users
    first_name := capitalize(user.name.split(" ")[0])
]

While it's possible to move the value of the first_name assignment directly to the comprehension term, it's arguably less readable, as it makes it harder to see that the form is a comprehension to begin with.

# Not recommended
first_names := [capitalize(user.name.split(" ")[0]) |
    some user in input.users
]

Configuration Options​

This linter rule provides the following configuration options:

rules:
  style:
    comprehension-tail-assignment:
      # one of "error", "warning", "ignore"
      level: error

Community​

If you think you've found a problem with this rule or its documentation, would like to suggest improvements, new rules, or just talk about Regal in general, please join us in the #regal channel in the Styra Community Slack!