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Alma-0 is a small imperative programming language that includes a limited number of features inspired by the logical programming paradigm. These additions support and encourage declarative programming and make Alma-0 an attractive option for programmers used to imperative languages when facing problems that involve search. The designers of Alma-0 claim that search-oriented solutions built with it are substantially simpler than their counterparts written in purely imperative or logical programming style.
Since the designers of Alma-0 wanted to create a distinct and substantially simpler proposal than previous attempts at integration of declarative programming constructs (such as automatic backtracking) into imperative programming, the design of Alma-0 was guided by the following four principles:
- the logic-based extension should be downward compatible with the underlying imperative programming language
- the logic-based extension should be upward compatible with a future extension that will support constraint programming
- the constructs that will implement the extension should support and encourage declarative programming.
- the extension should be kept small (nine new features have been proposed and implemented)
In fact, Alma-0 could be viewed not only as a specific and concrete programming language proposal, but rather as an example of a generic method for extending any imperative programming language with features that support declarative programming.
The feasibility of the Alma-0 approach has been demonstrated through a full implementation of the language (including a description of its semantics) for a subset of Modula-2.
The implemented features in Alma-0 include:
- use of boolean expressions as statements and vice versa
- a dual for the
FORstatement that introduces non-determinism in the form of choice points and backtracking
FORALLstatement that introduces a controlled form of iteration over the backtracking
- unification which, although limited to the use of equality as assignment, yields a new parameter-passing mechanism.
Imperative and logical programming modes
The Alma-0 designers claim that the assignment, which is usually shunned in pure declarative and logic programming, is actually needed in a number of natural situations, including for counting and recording purposes. They also affirm that the means of expression of such "natural" uses of assignment within the logic programming paradigm are unnatural.