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Information Contracts

Finitio tries very hard not to be yet another data language. In particular, it aims at integrating as smoothly as possible with existing technologies, in particular with programming languages and data exchange formats (e.g. JSON or YAML).

This interoperability is handled through so-called information contracts. In some respect, information contracts are the dual of axiomatic contracts, i.e. the dual of public behavioral APIs of software abstractions.

Information Contracts

For a given software abstraction, say a Color:

  • The axiomatic contract hides the internal representation in favor of a set of public behavioral methods to manipulate the abstraction (e.g. darkening and brightening the color),
  • The information contract hides the internal representation in favor of a set of public information representations of the abstraction (e.g. a RGB triple, an hexadecimal string).

The data types involved in the definitions of the information contracts are called information types, e.g. {r: Byte, g: Byte, b: Byte} (a tuple type). Finitio provides a rich type system dedicated at capturing those data types precisely, mostly because type systems of mainstream programming languages fail at providing good support for them.

Dressing & Undressing

In a more precise way, an information contract is actually a set of function pairs, such as:

# RGB information contract
dress   :: {r: Byte, g:Byte, b: Byte} -> Color
undress :: Color -> {r: Byte, g:Byte, b: Byte}

# HEX information contract
dress   :: String( s | s =~ /#[a-f0-9]{6}/ ) -> Color
undress :: Color -> String( s | s =~ /#[a-f0-9]{6}/ )

In other words, each public data representation of an abstraction comes with two (pure) functions that allow dressing the corresponding information type with the abstraction behavior, and undressing the latter the other way round.

If information contracts are best explained through abstract data types such as Color, the dress/undress principle is more general. It also allows explaining the interoperability of data exchange and programming languages. For instance, the interoperability with JSON could easily be explained as follows:

JSNumber = String( s | s =~ ... )
Integer  = <json> JSNumber \( s | ... ) \( i | ... )

This definition naturally defines the following information contract:

dress   :: JSNumber -> Integer
undress :: Integer -> JSNumber

The actual parsing/unparsing of JSON from/to text is generally done by dedicated third-party libraries, of course. The example only aims at showing that information contracts are a very general concept, that explains what is actually going on in practice.

Data Interoperability

According to the host programming language, however, the interoperability with exchange formats such as JSON is more of less complete. In Ruby, for instance, the interoperability is already pretty good. It can be explained as follows:

RbBoolean = <json> JSBoolean
RbString  = <json> JSString
RbNumeric = <json> JSNumber
RbHash    = <json> JSObject
RbArray   = <json> JSArray

Such a mechanism is already built into the Ruby standard library, and explains why working with JSON data is rather natural in Ruby (because Ruby classes at left above are first-class citizen for Ruby programmers). Among others, this allows Finitio-rb to be kept simple, and work with the Ruby type system only, delegating the interoperability with JSON to the usual parsing library. More work might be needed for other programming languages.

Observe, however, that Ruby/JSON interoperability is straightforward but actually biased towards JSON. The developer has absolutely no way of stating that some value must be a Ruby Integer, since the JSON specification does not distinguish between integers from reals. One aim with Finitio and its type system is to provide a way for developers to fix this, by also being able to specify more specific information contracts and have full control of them. In the example, one would like to express an information contract like the following one:

RbInteger = <json> JSNumber( s | s =~ /^[1-9][0-9]*$/ )

Contracts in Action

Dressing and undressing generally applies recursively, e.g. when involving collection and abstract data types. This provides the real ability of Finitio to dress and undress complex data involving many information contracts and many abstractions.

Consider the following Finitio system, i.e. for dressing sequences of tuples having a name attribute restricted to simple words:

Word = .String( s | s =~ /^[a-z]+$/ )
[{ name: Word }]

Dressing JSON data with Finitio-rb, for instance, involves the following contracts:

  • Dressing JSString to Ruby String (by the standard library)
  • Dressing Ruby String to Word (by Finitio-rb, returning a Ruby String)
  • Dressing Ruby Hash to Tuple (by Finitio-rb, returning a Ruby Struct)
  • Dressing Ruby Array to Seq (by Finitio-rb, returning a Ruby Array)

The concrete dressing result is implementation-dependent, as it involves the definition of the representation function Rep that binds Finitio types to types in the host language. The aim is not to define new host abstractions, e.g. classes, for every Finitio type defined in a system but rather to check that values conform to Finitio types and choose an idiomatic representation in the host language (see the parentheses). However, all those information contracts are actually involved in the dressing process and provide as many places to validate and coerce data in practice.