Compilation

The Riddl compiler performs several analyses which are described in this section and known collectively as compilation. Each of these analyses occurs in a compilation phase, as described in the following sections.

Riddl uses the excellent fastparse library by Li Haoyi. This phase parses the raw textual input to make sure it is syntactically correct. From that syntax, an abstract syntax tree (AST) is produced. Incorrect syntax leads to errors without further analysis.

If lexical analysis succeeds, an Abstract Syntax Tree (AST) is compiled as the internal representation of the RIDDL source input within the tools’ memory. Structural Analysis succeeds when the AST is constructed without error. The
AST represents the containment hierarchy of the input definitions.

To aid in reader comprehension, a certain way of using the RIDDL language is recommended. Optionally, the compiler can generate style warnings to indicate language specifications that deviate from that recommended style.

The Riddl AST is very flexible. It can accept a wide range of input, even input that doesn’t necessarily make logical sense. For example, suppose you wrote this:

entity MyLittlePachyderm is {
  state is {
    tusk is TuskDefinition
 }
}

This defines an entity type named MyLittlePachyderm which can be used as the pattern to instantiate many entities with this form. This entity is defined with a single state value, tusk that has the type TuskDefinition. Note that there is no definition of TuskDefinition as there is for the entity, state and tusk. Consequently, we don’t know the type of the tusk field so our specification is incomplete and would fail semantic analysis.

Semantic analysis, also known as validation, is the process of finding omissions, as described above, as well as:

• references to undefined things,
• references to existing things of the wrong type,
• constructs that may be confusing,
• definitional and logical inconsistencies
• and, etc.

The semantic analysis phase generates messages that identify the omissions and inconsistencies in the input specification. These validity issues typically stop the compiler from proceeding with translation because using an invalid input model tends to produce output that is flawed or less than useful.