Constructor FieldSelection

rascal-0.40.17

Synopsis

Select a field from a constructor by its field name.

Syntax

Exp . Name

Types

Exp	Name	Exp . Name
cons(T₁ L₁, T₂ L₂, ... )	Lᵢ	Tᵢ

Description

Field selection applies to constructors with named elements such as fields and keyword fields. Exp should evaluate to a tuple with field Name and returns the value of that field. Name stands for itself and is not evaluated.

Because the types of field names are known statically, field projection for constructors produces values of precise types. This is unlike FieldSelection where the resulting type is always value.

Constructor keyword fields have default expressions which can be dependent on each other and on the positional fields (by name). They are computed lazily upon request.

Examples

rascal>data Example = example(int key, str val="<key>");      ❶  
ok
rascal>t = example(42);
Example: example(42)
rascal>t.key = 2;
Example: example(2)
rascal>t.val
str: "2"
---
2
---
rascal>t.key += 2;
Example: example(4)
rascal>t.val
str: "4"
---
4
---

• ❶ val is a keyword field with a default value computed from the positional field key.

Benefits

• constructors with fields are a versatile way of modeling and storing hierarchical data