Alias/let syntax interpretation and use

[cueckoo]

Originally opened by @extemporalgenome in cuelang/cue#687

In the CUE language spec in the Aliases section (https://cuelang.org/docs/references/spec/#aliases) is the following syntactic form:

foo: X  // 4
X="not an identifier": 4

What is the purpose/meaning of the "not an identifier" part (it doesn't appear in the example output)?

Is it just a throwaway expression to syntactically distinguish normal fields from aliases when used in this way?

How would this be different from let declarations (which the tutorial describes as aliases, while the spec description of let does not use the term alias).

How are aliases and let declarations distinct from _# definitions?

Thanks

• Kevin

[cueckoo]

Original reply by @mpvl in cuelang/cue#687 (comment)

The purpose of the X in X="not an identifier": 4 is that it allows referencing the field. The field has a name that is not a valid identifier, so there would otherwise be no way to directly reference the field. For instance, the foo: X references that field and will evaluate to foo: "not an identifier": 4.

A let introduces a name for an expression that can be reused without it being part of the output.
So, where:

X=a: 4
b: X

would evaluate to

a: 4
b: 4

using a let

let X = 4
b: 4

evaluates to

b: 4

instead.

In general if you see a X = it introduces a new name for the element it precedes, without otherwise changing the meaning of this element. It can be used to work around a host of issues such as referring to fields that are otherwise invalid names, or just long, generated field names (such as in X="key(x)": int`) or elements that are otherwise hard or impossible to reference.

A let on the other hand, is used to hoist an often-repeated expression or just to break an expression into smaller chunks, without having to introduce a new field for that, for instance:

let byte = "[0-9a-x]{2}"
#hexIP: =~"\(byte).\(byte).\(byte).\(byte)"