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Requs Syntax

The scope definition document consists of types and methods (aka “use cases”). For example, there are two types (Visitor and Image) and one use case (UC3.2) defined in the snippet:

Visitor is "an originator of HTTP request".
UC3.2 where User (a user) deletes photos:
  "we will define it later...".
Image includes: content, name, and size.

Anything inside double quotes is considered as informal content, which we take “as is”, without any attempt to understand what it means. You can double quote anything (except double quotes, of course), and you can place informal texts in any place of the document. They are ignored during formal Requs analysis.

Let’s consider a bigger example of a type Image:

Image includes:
content as File "a binary content as defined by ISO-15948",
name as "a unique alpha-numeric name of the image",
size as "the total number of bytes in PNG content".

The semantic is the same – the type still contains three slots (PNG content, name and size). However in this example we added an informal explanation to every one of them. An as keyword separates the name of the slot and its invariant.

Pay attention to the dots, colons, semicolons and commas used above. Comma, semicolon and preposition and can replace each other. For example, the following declarations are semantically equivalent:

Image needs: content and name and size.
Image needs: content, name, size.
Image needs: content; name; and size.

By means of is/is a we enable a declaration of invariants on types. Invariant is a predicate, which is always true, no matter what happens with the system. In the example above it is always true that PNG content is a file with binary content (not a readable ASCII text). Nobody can break this invariant and put a textual information into this slot.

As you noticed, an invariant can be declared with an informal text, as we’ve done with size. Such a declaration means absolutely nothing and will be ignored. But it helps when you’re starting to develop the SRS document.

In general, informal texts play an important role during the initial development of a requirements document and during a preliminary system analysis. When you don’t know for sure how to define the information you have in a strict format – you should use informal texts. Later, when you have more information, you will replace them.

Requs is a case-insensitive language in all places except one – type names should use CamelCase Notation. Thus, File, ImageFile, and VeryBigImageFile are valid type names, while imagefile is just an English word.

Bigger Example

To put things together we should declare a use case, which is a step-by-step explanation of interaction between instances of types (so called objects), for example:

UC8.1 where User (a user) shares Image with User (a friend):
1. The user creates Image (a photo);
2. The user updates the friend "selecting one of his contacts";
3. The photo converts "to the right PNG format, 600x600 maximum";
4. The friend receives email using SmtpServer (a server);
5. The friend reads the photo "in his own web page".
UC8.1/1 when "the user exceeds the maximum possible number of photos":
1. Fail as "photos limit exceeded".
UC8.1/3 when "invalid format":
1. "We notify user about the problem";
2. Fail as "can't convert photo".

First line in this example is a declaration of a use case, which number is UC8.1. The use case has a signature, which differentiates it from all other use cases. It is not the ID of the use case, but the signature, which is important. This concept is very similar to function signature in programming. The signature of this use case is:

User shares Image with User

In order to include this use case into another one we should use this signature, filling it with particular objects. Objects start with article the. On their first occurrence they appear in round brackets and start with a or an.

In our use case there are three objects: the user, the friend and the photo. An object could be either received by a use case or created inside it. However, there is no grammar difference between objects received and objects created. We assume that an object is empty until anyone updates or creates it (more on this later).

There are seven main flows in this use case, and five alternative flows. Flows 1, 5, 6, and 7 instruct us to include other use cases that match the signatures provided and pass them the objects we have.


In the example above, one of the use case steps mentioned using operator:

The friend receives email using SmtpServer (a server).

This is similar to sending arguments to a method. In this line we’re calling a method “receives” on “the friend” object like this:


Instead of using you can use of or with. These three keywords are reserved and can’t be used as English words.


There are four use cases that are included by UC8.1. They have to be defined somewhere else in the document, otherwise the document will be semantically incomplete. But not all four are mandatory, because there is a number of elementary use cases, which are defined in the system, even if the document is empty. The elementary use cases are (CRUD requirements pattern):

Something creates something
Something reads something
Something updates something
Something deletes something

As you understand, something means “object of any type”. create means making/instantiating of a new object. read means reading of all and any slots of an object, and all their slots, etc. update means changing of values of slots of an object. delete means removing an object from a persistent storage.

Thus, a valid flow either points us to another use case defined somewhere else in the document, or points us to an elementary use case, or points us nowhere with an informal text (informal flow).


Besides that, a flow might have a special instruction, which we’ve seen in alternative flow UC8.3/1 and UC8.3/3. fail since means that a use case should be stopped at this point and the reason of this termination is explained right after the word since as an informal text.

Reason of failure is used by a parent use case, which included the current one. This is exactly what happens in UC8.3/3. We are waiting for a failure from we convert the photo, and we’re ready to accept a failure called invalid format. This approach is very similar to exception handling paradigm in object-oriented languages.

Scope Ambiguity

There is only one metric that tells us everything about the entire scope definition document. The metric is called “scope ambiguity” and is calculated like:

A = S / (S + M)

S stands for a total number of all informal flows, and M is a total number of CRUD-manipulators. Thus, if A equals to 0, the document is absolutely non-ambiguous, which is an almost impossible situation. In real world projects the task of a system analyst is to move A from 1 to 0.

Arity of Slots

A slot may have an “arity”, determined by a suffix attached to its name. There are four possible options:

User includes: name, address-s, and photo-s?.

-s means “one or many” or 1..* in UML.

-s? means “zero or many” or 0..* in UML.

No suffix means exactly one, or 1 in UML.

Composition vs Aggregation

A slot without a type means composition. A slot with a type means aggregation. In order to make a slot with a type of composition nature, you should use an exclamation mark after the type:

User includes: photo-s? as File!.


Any type can be an actor, just say so in your spec:

User is an actor.

Only actors can perform read operations.


Any use case may have attributes assigned to it. Attributes may be used, for example, for requirements prioritization.

:UC3.2 is a must.
:UC8 is delivered.
:UC2 is specified.

You can “seal” your statement with a hash code:

af63e2:UC3.2 is a must.

This hash code af63e2 is calculated from the content of UC3.2. If the content is changed, this statement becomes invalid and the entire document can’t be compiled any more.

More about it in Attributes and Seals.

Non-Functional Requirements

To any use case you can add a number of non-functional requirements (NFR), in the following format:

UC3.2/MTBF must "be 5 minutes on a standard equipment".
UC7/PERF must "be less than 500 msec per request".

After the name of the use case you put a forward slash and then a mnemonic name of a non-functional requirement. Then, you put “must be” and an informal text.

More about it in Non-Functional Requirements.

Markdown Pages

Sometimes you may need to add informal pages to your SRS document. For example, Vision, Business Case, wireframes, UI mockups, tables, research results, supplementary tables, etc. The syntax is simple:

Vision: """
Any text you wish, in Markdown format.

Page name should be in CamelCase, as well as type name.

Read on Markdown syntax.