Paradigm multi-paradigm: extensible, object-oriented, imperative, structured, generic, reflective
Designed by Thomas Mertes
First appeared 2005 (2005)
Stable release
2016-10-31 / October 31, 2016 (2016-10-31)
Typing discipline static, strong, safe, nominative, manifest
OS Cross-platform: BSD, Linux, OS X, Unix, Windows
License GPL, LGPL (for the runtime library)
Filename extensions .sd7, .s7i
Major implementations
open source reference implementation
Influenced by
Pascal, Modula-2, Ada, ALGOL 68, C, C++, Java

Seed7 is an extensible general-purpose programming language designed by Thomas Mertes. It is syntactically similar to Pascal and Ada. Along with many other features, it provides an extension mechanism.[1] Seed7 supports introducing new syntax elements and their semantics into the language, and allows new language constructs to be defined and written in Seed7.[2] E.g.: programmers can introduce syntax and semantics of new statements and user defined operator symbols. The implementation of Seed7 differs significantly from that of languages with hard-coded syntax and semantics.


Seed7 supports the programming paradigms: imperative, object-oriented (OO), and generic. It also supports features such as call by name, multiple dispatch, function overloading, operator overloading, exception handling and arbitrary-precision arithmetic.

Major features include:

Several programming language concepts are generalized:

The Seed7 project includes both an interpreter and a compiler. The interpreter starts programs very quickly. This supports fast program development. The compiler uses the parser and reflection interfaces from the run-time library to generate a C program, which is subsequently compiled to machine code. Compiled Seed7 programs can have similar performance as C programs.


Seed7 has many libraries, which cover areas like containers, numeric functions, lexical analysis, file manipulation, networking (sockets, Transport Layer Security (TLS/SSL), Hypertext Transfer Protocol (HTTP), HTTP Secure (HTTPS), File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP), etc.), graphics, pixmap and vector fonts, database access (MySQL-MariaDB, SQLite, PostgreSQL, Oracle, Open Database Connectivity (ODBC)), Common Gateway Interface (CGI) support, data compression, character encoding, time and date handling, XML processing, message digests and more. This lowers the need to use operating system features and third party libraries directly. Seed7 libraries[3] contain abstraction layers for hardware, operating system and third party libraries, e.g., graphic and database libraries. In other words, no changes are needed to move Seed7 programs between different processors or operating systems.


Seed7 is based on MASTER, an extensible programming language described in the diploma and doctoral theses of Thomas Mertes.[4][5] Most of the original ideas of MASTER, such as user defined statements and operators, can be found in Seed7. A precompiler, to translate MASTER to Pascal, was proposed, but unimplemented, in the original project. In 1989, development began on an interpreter for MASTER, named HAL. In 2005, the MASTER and HAL projects were released as open source under the Seed7 project name. Since then new versions have been released every two or three weeks. As of version 2013-09-08 the Seed7 project contains more than 300,000 source lines of code and several hundred pages of documentation.

Extension mechanism

An extension includes two parts: a syntax definition, giving a template for the new syntactic form, and a standard Seed7 function, used to define the semantics.[1]

Syntax definition

The syntax definition uses the Seed7 Structured Syntax Description (S7SSD). A S7SSD statement like

$ syntax expr: .(). + .()  is -> 7;

specifies the syntax of the + operator. The right arrow -> describes the associativity: Binding of operands from left to right. With 7 the priority of the + operator is defined. The syntax pattern .(). + .() is introduced and delimited with dots (.). Without dots the pattern is () + (). The symbol () is a nonterminal symbol and + is a terminal symbol. The S7SSD does not distinguish between different nonterminal symbols. Instead it only knows one nonterminal symbol: ().

Semantic extension

The definition of the + operator for complex numbers is just a function definition:

const func complex: (in complex: summand1) + (in complex: summand2) is func
    var complex: sum is complex.value;
  begin := +; := +;
  end func;

Further reading

"To the best of our knowledge, among all these languages only the Seed7 programming language supports the introduction of new syntax and their semantics into the language."
"In terms of language extensibility, Seed7 goes beyond CoreASM as it allows new language constructs to be defined using the Seed7 language itself."


  1. 1 2 Zingaro, Daniel, "Modern Extensible Languages", SQRL Report 47 McMaster University (October 2007), page 16.
  2. Abrial, Jean-Raymond and Glässer, Uwe, "Rigorous Methods for Software Construction and Analysis", ISBN 978-3-642-11446-5, Springer, 2010, page 166.
  3. Seed7 libraries
  4. Mertes, Thomas, "Entwurf einer erweiterbaren höheren Programmiersprache", Diploma thesis Vienna University of Technology (1984).
  5. Mertes, Thomas, "Definition einer erweiterbaren höheren Programmiersprache", Doctoral thesis Vienna University of Technology (1986).
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