Let transform

This topic tells you about the Application Accelerator Let transform in Tanzu Application Platform (commonly known as TAP).

The Let transform wraps another transform, creating a new scope that extends the existing scope.

SpEL expressions inside the Let can access variables from both the existing scope and the new scope.

Variables defined by the Let should not shadow existing variables. If they do, those existing variables won’t be accessible.

Syntax reference

type: Let
symbols:
- name: <string>
  expression: <SpEL expression>
- ...
in: <transform> # <- new symbols are visible in here

Execution

The Let adds variables to the new scope by computation of SpEL expressions.

engine:
  let:
  - name: <string>
    expression: <SpEL expression>
  - ...

Both a name and an expression must define each symbol where:

• name must be a camelCase string name. If a let symbol happens to have the same name as a symbol already defined in the surrounding scope, then the local symbol shadows the symbol from the surrounding scope. This makes the variable from the surrounding scope inaccessible in the remainder of the Let but doesn’t alter its original value.

• expression must be a valid SpEL expression expressed as a YAML string. Be careful when using the # symbol for variable evaluation, because this is the comment marker in YAML. So SpEL expressions in YAML must enclose strings in quotes or rely on block style. For more information about block style, see Block Style Productions.

Symbols defined in the Let are evaluated in the new scope in the order they are defined. This means that symbols lower in the list can make use of the variables defined higher in the list but not the other way around.

See also

• Combo provides a way to declare a Let scope and other transforms in a short syntax.