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MANOOL is Not an Object-Oriented Language!”

MANOOL — Practical Language with Universal Syntax and Only Library-Level Features (Except One)

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First, I have to make two disclaimers:

In this article I do not intend to propose “the next greatest” minimalist Turing-complete programming language; we already have at least two of them: Iota and Jot (based on combinatory logic)1. These are extreme, “esoteric” examples that show that language minimalism itself may be a useless, even absurd design goal from the practical standpoint. So, this article focuses on something more practical — I rather try to show that (a kind of) minimalism may help to achieve other attractive goals.

This article talks about simple, cost-effective solutions in the spirit of MANOOL design, easy to implement and easy to master, although they may be a bit incomplete (which is aligned with the Pareto principle, also known as the 80/20 rule). Thus, I do not pretend to make scientifically rigorous claims here since ultimately, simplicity is difficult to formalize.

In other words, although I strove to align the article and the entire MANOOL design with the established theoretic computer science formalisms and terminology, this article is not about computer science but rather software engineering arts. However, it does talk (hopefully, in an entertaining form) about how some phenomena and connections between them predetermine some key aspects of the architecture of the programming language MANOOL (thus, this article is hopefully neither a goal-oriented technical manual nor a meaningless advertisement).

Also, to avoid any misunderstandings, let's agree on some important definitions that hold throughout this article:

A syntax of a formal language (such as a programming language) is a set of rules that determine how to discover a syntactic structure of any phrase in that language, where a syntactic structure of a phrase is a tree-like (i.e., hierarchical) structure induced on that phrase (in the form of a character string) that enables formulation of the meaning of the phrase (or a constituent phrase thereof) by means of a straightforward recursive definition.2 Sometimes different syntaxes may adequately describe the same programming language, and the syntax is normally specified by a context-free grammar plus lexical rules.

A semantics of a formal language is a set of rules that determine how to discover the meaning of any phrase in that language.

(See Definitions)

Note that these definitions may be at odds with your intuition and may slightly differ from the corresponding definitions as they appear in some other contexts (such as specifications of other programming languages, theoretic and applied linguistics, etc.).

And now, let's start …

General

The problem — feature bloat

Have you ever said to yourself?:

This new version of my favorite programming language is great, but why on earth did they introduce the feature X? I don't need it. The language has become now more bloated than ever!

or

This new version of my favorite programming language is great, but it would be even better if they introduced the feature Y.

See the pattern? — In one case you're complaining about the introduction of a new feature X, whereas in another case you're doing (exactly) the opposite: requesting a new feature Y (which might happen to be the feature X from another user's standpoint).

And since we all have different problems at hand and therefore different needs, in the course of programming language evolution, satisfying the needs of some language users eventually implies disappointing other users (those who request new features as well). So, can we break this vicious circle?

Summary — solution in MANOOL

The answer of MANOOL to the above question is “Yes, we can!”. For instance, in MANOOL there are

instead, all of the above are just library features (like, say, standard mathematical functions). You normally import them explicitly from a standard library module into the current binding environment in order to be able to use them, but this is completely optional — with the same success, you can write and import from your own similar modules instead, and as the language evolves, different standard library versions can be easily provided (based on the same code base). In the same way, several language dialects and/or sublanguages (including domain-specific ones) can be provided in the form of library modules and can even be mixed in the same region of the program unit (provided all naming conflicts, if any, are resolved somehow) or used separately from within different regions.

Incidentally, the only built-in, predefined construct in MANOOL is extern, which is used to reference some external entity (i.e., either a first-class value or a special, non-value entity, such as a standard library module, or a special, non-value entity that the keyword if denotes). Actually, there are also some core features, like

however, they

Of course, in practice, any such system inevitably has its limitations and scope of applicability, since every programming language is a result of many design trade-offs.3 Also, this is not a completely new approach — there are some related studies in theoretic computer science, and there were a few intents to design other programming languages with library-only functionality (sometimes, but not always, qualified as extensible).4 Unfortunately, no such language can be considered mainstream nowadays (and I do aim MANOOL to be a practical tool and maybe to become kind of mainstream some day); thus, I invite you to discover yourself whether the approach and especially the limitations of MANOOL make sense for you.

The MANOOL approach involves three aspects:

Further in this article, I explain what a universal syntax is and provide a few examples of programs in MANOOL for illustration purposes, although unfortunately, the article has no room for a detailed description of the MANOOL syntax as such, its advantages, drawbacks and/or the rationale (apart from a simple reproduction of the grammar). Also, I provide some examples that support the above claims about feature composability and modularity of the language but do not provide a detailed description of the MANOOL module system. Both topics are two whole new stories that deserve separate articles, which I'll publish in the future, so if you are curious, stay tuned …

I also touch in this article the closely related subjects of homoiconicity, metaprogramming, and syntactic macros (for the case of MANOOL).

Further motivation

Let's see what may happen if we just put up with language bloat …

Let's assume that C++98 is an extension of C. Remember the famous issue with the C++ grammar? — What does the following declaration in C++ mean?:

t1 a(t2());

Is a declared as an object of type t1 and initialized with the value of a value-initialized object of type t2, or is it a function returning a result of type t1 and taking as an argument a pointer to a function without parameters returning a result of type t2?

The above code exposes a parsing conflict. Of course, the C++ specification still tells us that a is a function, but this is because the specification has additional conflict-resolution provisions; in fact, no (Chomsky-style)5 grammar can unambiguously capture the distinction between the two situations (a is an object vs. a is a function). And even though we can always resolve parsing conflicts in some way, I argue that the resulting language may become then needlessly irregular, inconsistent, complex, and unintuitive due to additional (ad hoc) conflict-resolution rules.

But how often the above situation arises in practice? — Well, the C++ standard committee made this mistake in C++98 and suggested a fix in C++11 via the uniform initialization notation. Also, while I was playing around with LALR(1)-grammars for MANOOL, I realized that it was too easy to accidentally introduce grammar ambiguities (especially if we are short of terminal symbols to play with). It is even more easy to come to a grammar that is unambiguous but does not fall into the (cost-effective) LALR(1) class (which is undesirable).

The above illustrates that we should not ignore language bloat (which is inherently unavoidable) since it may cause real problems and may be harmful if we do not mitigate the related issues somehow. Besides, the situation would be even worse if instead of language evolution and extensibility, we were talking about modularity and composability (out of sublanguages).

State of the art

There are usually two kinds of features in a programming language:

As shown above, syntactic conflicts are tricky to avoid and tricky to manage. On the other hand, naming conflicts can be more plausibly resolved via name spacing and/or aliasing. So, what if we could unify both kinds of features and provide all of them just using names (or as they say, bind them to names)?

There is at least one language that excels at such unification, Kernel,6 but the problem with Kernel is that its computation model seems to be almost inevitably inefficient because in Kernel actions that are otherwise performed once, during what would be called a compilation phase,7 are performed repeatedly, every time an expression is evaluated (in the course of program execution).

A simple solution to that problem consists in designing a language in such a way that such activities are explicitly factored out into a separate compilation phase. And here we come to a translation/execution scheme that resembles in many ways the usual evaluation model of any dialect of Lisp.8 Compared to Kernel, a (small) price is that in some cases names are now bound to second-class entities (which cannot be freely operated with at run-time), but we still enjoy a unified approach (at 80%).

Note that there also exist quite a few studies about general context-free grammar composability, although MANOOL is based on a simpler approach to the problem of feature composition (see also Background theory).

Example

Almost every working example from the MANOOL tutorial contains near its beginning an instance of the extern construct (i.e., {extern "..."}), e.g.:9

{{extern "manool.org.18/std/0.3/all"} in Out.WriteLine["Hello, world!"]}

where the standard library module is identified by a special external entity path specified as a compile-time expression that evaluates to a string (i.e., "manool.org.18/..."). As you may note, a global name hierarchy is proposed based on the global Internet domain name system, which is a convention for some other programming languages (here, 18 means the year of manool.org domain registration — 2018).

The importing part is ... in ..., which is used here to “apply” the referenced external module (a standard library module in this case) to the body expression after in, making that expression to be compiled in the binding environment augmented with all name bindings the module provides. When evaluated, the body expression will transmit the "Hello, world!" string to the standard output stream of the current process.

In fact, the only standard library member used in the previous example is Out, which represents the standard output. WriteLine here is not a library-supplied feature at all since it denotes a so-called polymorphic operation (invoked in this case on Out), which in MANOOL is deliberately the same thing as a value of type Symbol (specified literally here), which can be applied to arguments as a procedure or any other procedure-like value.

Mechanisms and Techniques

Background theory

Together with the problem of programming language extensibility, let's consider also a broader problem of language modularity and composability (out of sublanguages). First, here are some quick well-known facts from the programming language theory:

  1. In order to be able to describe (in finite terms) the meaning of an infinite number of phrases (programs), the meaning of a phrase φ is determined as some composition M of meanings of its constituent phrases φi with respect to some given set of phrase-formation operations: [[φ]] = M([[φ0]], [[φ1]], …, [[φn-1]])10 — divide, and you'll master the infinity!

  2. Thus, a phrase is either an elementary phrase (pi) or a composition of its constituent phrases φi with respect to the phrase-formation operations mentioned above.

  3. In case a phrase is a character string, the elementary phrases (ai) consist of individual characters, and the phrase-formation operation is just the string concatenation: α = α0α1αn-1.

  4. In case a phrase is given in the form of a term (a tree) instead, the elementary phrases are nullary terms (ti), and the phrase-formation operations are term constructors: τ = f(τ0, τ1, …, τn-1) for each non-nullary functor f.

  5. The fundamental difference between the two phrase spaces (string and term) is that in contrast to term decomposition, string decomposition is ambiguous with respect to its phrase-formation operation alone (i.e., the concatenation operation applied in the process of phrase derivation).

  6. For that reason, (syntactic) (de)composition of source code is often described by an (attributed) unambiguous context-free grammar, and the meaning of programs is ultimately described as the meaning of terms (i.e., concrete or abstract syntax trees) instead.11

  7. The space of Lisp data (i.e., Lisp-style pairs plus atoms) constitutes a term algebra and is homomorphic to any other term algebra; thus, any claims about term algebras are automatically applicable to Lisp data and vice versa. The homomorphism is due to such mappings as

    f(τ0, τ1, …, τn-1) — (f, τ0, τ1, …, τn-1) — (f τ0 τ1τn-1) = (f . (τ0 τ1τn-1)) — (f, (τ0 τ1τn-1)) — pair(f, (τ0 τ1τn-1))

    Using Lisp data can be considered as just the most basic storage strategy for efficient term manipulation, and alternative data structures may be used as well.

  8. Although some aspects of AST (abstract syntax tree) validity and interpretation can be described themselves by (regular) tree grammars (which is a norm in MANOOL specification), such grammars are not really essential for the principle from the fact (1) to work (that is, for recursive definitions of meaning to do the job, term structure alone is already sufficient).

Now, whereas some authors argue that arbitrary context-free grammars are inherently anti-modular and non-composable (as the language evolves it becomes increasingly easy to introduce grammar ambiguities — think C++) , others have studied grammar composability and proposed methods to adjust a union of two or more grammars for the final grammar to make sense and be usable to describe syntax. However, according to its design philosophy, MANOOL takes an arguably simpler route: for any two languages L and L' to be composable, they both should be defined by using the same context-free grammar (thus eliminating the problem of grammar composability altogether).

Universal syntax

Ideally, the universal syntax (as it is at any moment) suits any future needs in a programming language and therefore should change very, very seldom, if at all. S-expressions (in any Lisp-family programming language) are described by such a syntax, and whereas MANOOL uses a slightly different notation, the MANOOL syntax has the same dynamics as in Lisp(s).12

A universal syntax is specified by a universal context-free grammar plus lexical rules (universal as well). Different attributed context-free grammars may adequately do the job of syntactic decomposition for the same programming language, but for the idea of a universal grammar to work as well as possible, the grammar should be as general as possible among those grammars. For instance,

The grammar of MANOOL was formulated empirically (by trial and error) and is intended to cover approximately 80% of practical needs,13 although given the above design principles and the desire to restrict the basic character set to good-old ASCII, the design space was in fact narrow. As a result, for an untrained eye the MANOOL notation (and my default rules of code indentation) may look unfamiliar or even weird; personally, I had a lot or resistance at first but got used surprisingly quickly.14 One of the sources of inspiration for me to use an unusual notation (if I had to at all) was Smalltalk.

Funnily enough, a universal syntax is not even mentioned among the MANOOL language design goals and has never been, but it has quickly become a part of the implementation strategy; designing a new practical programming language requires a lot of experimentation, and thus it would be unproductive to have to deal with context-free grammar changes every time a new feature is added, removed, or altered.

Compared to the “Hello, world!” example presented above, the following example uses

{ {extern "manool.org.18/std/0.3/all"} in
: let rec
  { Fact =
    { proc { N } as
    : if N == 0 then 1 else
      N * Fact[N - 1]
    }
  }
  in
  Out.WriteLine["Factorial of 10 is "; Fact[10]]
}

This is a version of a classic “Hello, world!” analog for functional languages. I modified the original version to turn it into a complete, runnable program, which writes to the standard output the factorial of 10 in decimal form by using a recursive definition of the factorial function.

Note that in its particular context above, (=) does not denote a polymorphic operation; it is just an “abstract operator” that serves to formulate name-meaning associations syntactically (i.e., a = b).

When translating the above code into an internal run-time representation, the MANOOL syntactic analyzer (also known as the parser) first transforms the code into an intermediate representation (that is, an AST), which incidentally, corresponds to the following equivalent code in MANOOL:

{ {extern "manool.org.18/std/0.3/all"} in
  { let rec
    { { (=) Fact
        { proc { N } as
          { if {(==) N 0} then 1 else
            {(*) N {Fact {(-) N 1}}}
          }
        }
      }
    }
    in
    {WriteLine Out "Factorial of 10 is " {Fact 10}}
  }
}

where any construct {e0 e1en-1} represents (in textual form) an internal AST node with n successors e0, e1, …, en-1, and almost everything else represents leaf nodes: extern, "manool.org.18/...", Fact, (=), (-), 1, etc.

Note that instead of talking about tree nodes, we might talk about Lisp data (and consequently, about Lisp-style lists, pairs, and atoms) as well, but due to some deep technical and philosophical reasons (which are beyond the scope of this article), we conform with just (unlabeled) internal nodes with n successors (or equivalently, n-element tuples or arrays) plus leaf nodes, which in practice gives us the same advantages. For the same reasons, the MANOOL core does not support improper lists on the AST level and thus does not have any analog of (S-expression) dotted pair notation.15

In somewhat general terms, the following identities hold in MANOOL on the AST level:

Note how applicants (i.e., a in the first identity) and operators (e.g., !, +) correspond to leading (head) successors of internal AST nodes. This is important because the head successor determines in a uniform way how the whole node is to be translated into an internal run-time representation (see below).

As you may note, the MANOOL syntax has syntactic sugar that acts abstractly and independently of what a[b0; b1;; bn-1] and a + b actually mean. Some possible interpretations of MANOOL syntactic constructs are

In all of the above cases, the MANOOL parser is to be reused, which makes up a foundation for a rich but uniformly accessible toolset and emphasizes the relation of the above listed areas to abstract algebra(s), so to speak.

The module system

In classic modular programming languages, such as Modula-2 and Ada, a module definition may serve for a combination of one or more purposes at a time:

In MANOOL, instead, a number of separate and mutually orthogonal features are available that cover the above mechanisms and can be easily combined:

Let's replace in the first factorial example the usual extern expression with a module constructor {... in: export ...} that describes our own module exporting only four features:

{ { {extern "manool.org.18/std/0.3/all"} in -- my local module
  : export let; proc; if; Out
  } -- end of local module
  in
: let rec
  { Fact =
    { proc { N } as
    : if N == 0 then 1 else
      N * Fact[N - 1]
    }
  }
  in
  Out.WriteLine["Factorial of 10 is "; Fact[10]]
}

This corresponds to the idea of local modules in Modula-2 or nested packages in Ada, but in this example the module does not even have a name and is imported immediately instead of being bound to some name or made an external entity. This is useful for selective import of features and for information hiding in the spirit of the local ... end construct of Standard ML.

Now, to illustrate the idea of a fully-replaceable standard library proposed above, let's create two text files and place them in the same directory:

Although non-value entities are second-class entities, they are quite flexible otherwise — they can be denoted by complex non-value expressions and bound to names (aliased), statically (that is, at compile-time), for example:16

{ {{extern "manool.org.18/std/0.3/all"} in let}
  { myLib = {{extern "manool.org.18/std/0.3/all"} in: export let; proc; if; Out} } in
: {myLib in let} { lambda = {myLib in proc} } in
: {myLib in let} rec
  { Fact =
    { lambda { N } as
    : {myLib in if} N == 0 then 1 else
      N * Fact[N - 1]
    }
  }
  in
  {myLib in Out}.WriteLine["Factorial of 10 is "; Fact[10]]
}

The expression {myLib in let} denotes the feature let from the module myLib (also available as let from the standard library), and the lambda head keyword in the expression {lambda ... as ...} behaves like proc.

Note that the keywords in, as, rec, then, and else above do not refer to “features”, and therefore they cannot be aliased or turned (on their own, separately and independently) into members of a module, unfortunately. Incidentally, this is one of the limitations of MANOOL I am talking about above; I argue, however, that the MANOOL solution is still aligned with the 80/20 rule.

Connection with S-expressions and Lisp(s)

As you may note, under certain circumstances the notation of MANOOL closely resembles the notation of S-expressions (in any Lisp-family programming language) where instead of parentheses (()) braces ({}) are used and on the other hand some symbols (namely, non-alphanumeric ones) appear enclosed in parentheses. With “proper” indentation the S-expression equivalent of both factorial examples presented above looks like:

((extern "manool.org.18/std/0.3/all")
   in
   (let rec ((= Fact
                (proc (N)
                  as
                  (if (== N 0)
                      then
                      1
                      else
                      (* N (Fact (- N 1)))))))
     in
     (WriteLine Out "Factorial of 10 is " (Fact 10))))

In contrast to the case of Lisp-family programming languages, here, in some cases elements (keywords) like in, rec, as, then, and else, are essential for meaning and thus cannot be stripped off without introducing ambiguity. However, an equivalent program in Scheme has an almost identical syntactic structure:

(letrec ((Fact
          (lambda (N)
            (if (= N 0)
                1
                (* N (Fact (- N 1)))))))
  (display "Factorial of 10 is ")
  (display (Fact 10))
  (newline))

Some people love S-expressions as a programming language syntax, whereas others hate them, and people often have strong opinions and/or preferences in respect of programming language notation, since it lies on the surface and stays in constant contact with language users. Thus, I suspect the former are going to criticize me for not using the (concrete, surface) syntax of S-expressions for MANOOL, especially having such a good opportunity.17 For now, just for now, however, please regard the concrete syntax as a matter of personal choice that is irrelevant to the subject of this article. As I have promised, I'll publish a separate article about the MANOOL syntax and its rationale.

Homoiconicity, metaprogramming, syntactic macros

Homoiconicity is the ability of a programming language to express programs that process other programs in a very specific, Lisp-like, way. Such languages are called homoiconic, and hoiconicity has to do with metaprogramming and syntactic macros. Homoiconicity, in particular, requires that ASTs could be manipulated programmatically. Incidentally, MANOOL is a homoiconic language (and that is why its attributed context-free grammar has semantic evaluation functions metacircularly specified as code in MANOOL).

This simple but a bit contrived example in MANOOL demonstrates that we can translate if ... then and if ... then ... else into, say, Spanish by defining a syntactic macro (but making it available in a limited region only):

{ {extern "manool.org.18/std/0.3/all"} in
: let
  { si =
    { macro
    : proc { F } as
    : if (Size[F] >= 7) & (F[2] == entonces') & (F[4] == si') & (F[5] == no') then
      {array of if# F[1] then' F[3] else'} + F[Range[6; Size[F]]] -- {si ... entonces ... si no ...}
      else
    : if (Size[F] >= 4) & (F[2] == entonces') then
      {array of if# F[1] then'} + F[Range[3; Size[F]]] -- {si ... entonces ...}
      else
      Nil -- fallback
    }
  }
  in -- you can use {si ...} hereinafter, up to the end of the compound expression, except where shadowed
: let rec
  { Fact =
    { proc { N } as
    : si N == 0 entonces 1 si no -- hablo espanol
      N * Fact[N - 1]
    }
  }
  in
  Out.WriteLine["Factorial of 10 is "; Fact[10]]
}

Don't worry if you don't understand this example fully right now — just make sure that you can see for yourself from the code that this is possible (but stay tuned if you are intrigued by this example and expect a future article with a detailed explanation of it ;-)

Translation Scheme

In this section I explain how it all works in MANOOL by citing relevant parts of the official specification (so, please bear with me).

This section presents the attributed context-free grammar and contains also quite a lot of advanced MANOOL code, but unfortunately, this article has no room for detailed explanations. But don't be afraid — this material is included merely for completeness and to better connect the ideas expressed in this article with the MANOOL language reality — you don't have to understand this material fully in order to appreciate these ideas (though, it may be helpful). Also remember that you can always consult the MANOOL Tutorial and/or the official MANOOL Specification for more information.

Compilation phases

So, according to the official MANOOL specification:

To figure out the meaning of a form that makes up a program unit, the abstract machine transforms (compiles) the contents of the source file into an internal run-time representation, called run-time code.

A three-stage translation (i.e., compilation) scheme is suggested for the abstract machine:

  • lexical analysis — The input string of characters is split into lexical elements (lexemes), whose meaning is then encoded in left-to-right order as a sequence of tokens. Note that in practice, whatever internal syntactic structure of individual lexemes is devised, it is generally unimportant for determination of their meaning; rather, the lexical syntax is used for their sheer classification.

  • syntactic analysis — The string of terminal symbols that corresponds to the sequence of tokens resulting from the previous compilation phase undergoes a syntactic analysis guided by a context-free grammar, which ultimately yields an abstract syntax tree (AST) encoded as a MANOOL (semantic) value. Note that in contrast to lexical analysis, here syntactic structure is essential for correct interpretation of source code.

  • semantic analysis and code generation — The form and consistency (e.g., the presence and placement of certain keywords) of the AST resulting from the previous compilation phase are checked, and finally, the run-time code is produced. Note that no new structural features are to be exposed on this stage, or they would at least reflect closely those of the AST.

(See The Abstract Machine and Translation Overview)

Grammar productions

The context-free grammar of MANOOL is in fact quite simple (having a total of only 34 productions):

<start>S  ->  <datum>S
<datum>S  ->  <datum'>S
<datum>S  ->  <datum'>S[0] <equ op>L[1] <datum'>S[2]  [[ MakeList[{array of A[1]; A[0]; A[2]}] ]]
<datum'>S ->  <simple>S
<datum'>S ->  <simple>S[0] <rel op>L[1] <simple>S[2]  [[ MakeList[{array of A[1]; A[0]; A[2]}] ]]
<simple>S ->  <term>S
<simple>S ->  <simple>S[0] <add op>L[1] <term>S[2]  [[ MakeList[{array of A[1]; A[0]; A[2]}] ]]
<term>S   ->  <factor>S
<term>S   ->  <term>S[0] <mul op>L[1] <factor>S[2]  [[ MakeList[{array of A[1]; A[0]; A[2]}] ]]
<factor>S ->  <prim>S
<factor>S ->  <pref op>L[0] <factor>S[1]  [[ MakeList[{array of A[0]; A[1]}] ]]
<prim>S   ->  <prim'>S
<prim>S   ->  <prim>S[0] "["L <args>S[2] "]"L  [[ MakeList[{array of A[0]} + A[2]] ]]
<prim>S   ->  <prim>S[0] "."L <prim'>S[2] "["L <args>S[4] "]"L  [[ MakeList[{array of A[2]; A[0]} + A[4]] ]]
<prim>S   ->  <prim>S[0] <post op>L[1]  [[ MakeList[{array of A[1]; A[0]}] ]]
<prim'>S  ->  <lit>L[0]  [[ A[0] ]]  |  "("L <op>S[1] ")"L  [[ A[1] ]]
<prim'>S  ->  "{"L <list>S[1] "}"L  [[ A[1] ]]  |  "("L <datum>S[1] ")"L  [[ A[1] ]]
<op>S     ->  <equ op>L | <rel op>L | <add op>L | <mul op>L | <pref op>L | <post op>L
<args>S   ->  <args'>S  |  ""  [[ MakeList[{array}] ]]
<args'>S  ->  <datum>S[0] <args>S[1]  [[ MakeList[{array of A[0]} + A[1]] ]]
<args'>S  ->  <datum>S[0] ";"L <args'>S[2]  [[ MakeList[{array of A[0]} + A[2]] ]]
<list>S   ->  <list'>S  |  ""  [[ MakeList[{array}] ]]
<list'>S  ->  <datum>S[0] <list>S[1]  [[ MakeList[{array of A[0]} + A[1]] ]]
<list'>S  ->  <datum>S[0] ";"L <list'>S[2]  [[ MakeList[{array of A[0]} + A[2]] ]]
<list'>S  ->  <datum>S[0] ":"L <list'>S[2]  [[ MakeList[{array of A[0]; A[2]}] ]]

where MakeList is some (pure) function, and for any array A of ASTs, the following condition is met:

MakeList[A].IsList[] & (Size[MakeList[A]] == Size[A]) & {for {E1 = MakeList[A]; E2 = A} all E1 == E2}

(See The context-free grammar for information on the notation and Lexical Structure for information on lexical elements and lexical categories)

Compiler dispatcher

The official MANOOL specification has the following to say about the third compilation stage:

Run-time code, which is the output of the semantic analysis and code generation translation phase, is represented by a compiled entity, which internally (in its turn) may contain other compiled entities. Incidentally, compiled entities are also what identifiers are bound to in a binding environment.

A kind of object-oriented approach is used in this specification — compiled entities are considered to be active agents (represented by a MANOOL value/object for the purposes of the metacircular description) capable of providing services whenever the abstract machine asks them to.

The abstract machine has a (very) simple and compact semantic analysis and code generation core dispatcher. To compile an expression, the dispatcher first determines what kind of AST represents it:

  1. If it is a symbol (explicitly) bound to a compiled entity in the active binding environment, that compiled entity becomes the result of compilation.

  2. Otherwise, if it is an integer, a string, or a symbol, and the symbol starts with a character other than an ASCII lowercase letter (a thru z), the abstract machine constructs a compiled entity that represents a literal value.

  3. Otherwise, if it is a compound expression, the dispatcher compiles (recursively) the head subexpression and then asks the resulting compiled entity to compile the whole expression.

  4. Otherwise, if it is a special MANOOL value that encapsulates a compiled entity, which is produced by a #-expression (and for which the predicate IsCode returns True), then the encapsulated compiled entity becomes the result of compilation.

  5. As a fallback, the dispatcher reports an error as appropriate.

Note that checking the presence and placement of keywords (such as then, else, do, etc.) is performed, if needed, by compiled entities rather than directly by the dispatcher. Also, a compiled entity, when asked to compile an expression, may in turn call the dispatcher for subexpressions and possibly specify a different active binding environment to compile in.

(See Semantic Concepts and Compiler Dispatcher)

Metacircular specification

In this last subsection I present hypothetic code in MANOOL that serves a double purpose:

For the purposes of illustration, let's assume that a compiled entity recognizes the following polymorphic operations (beyond the standard ones):

  • IsRvalue — tell whether the compiled entity corresponds to an r-value expression

  • IsLvalue — tell whether the compiled entity corresponds to an l-value expression (which shall imply a positive answer to the above question as well)

  • Compile — given an active binding environment represented by a mapping SymTab, compile the specified compound expression (whose head subexpression corresponds to the compiled entity) to produce as a result another compiled entity

  • Execute — given an evaluation context Ctx, evaluate the expression the compiled entity represents and produce a resulting value (applicable only in case of r-value expressions)

  • ExecIn — given an evaluation context Ctx and a value Val, store the value into the location the compiled entity represents (applicable only in case of l-value expressions)

  • ExecOut — given an evaluation context Ctx, move out the current value from the location the compiled entity represents (applicable only in case of l-value expressions) to produce as a result the value moved out

The core dispatcher algorithm described above may be specified more formally using the following metacircular description (in MANOOL):

{ let rec
  { Compile =
    { proc { Form; SymTab } as
    : if IsSym[Form] & SymTab.Exists[Form] then SymTab[Form] else -- bound identifier
    : if IsInt[Form] | IsStr[Form] | IsSym[Form] &
      { do True after
      : if (Str[Form] <> "") & (Str[Form][0] >= "a"[0]$) & (Str[Form][0] <= "z"[0]$) then
      : signal CompileError with "unbound keyword (nested in this context)"
      }
      then -- literal value
      { let { _Form } in
      : object { _Form = Form } with
        IsRvalue = {proc {_} as True}
        IsLvalue = {proc {_} as False}
        Compile' = CompileApply
        Execute  = {proc {Self; _} as Self[_Form]@}
      }
      else
    : if IsList[Form] & (Size[Form] <> 0) then Compile[Form[0]; SymTab].(Compile')[Form; SymTab] else -- compound expression
    : if IsCode[Form] then Form[_Entity]@ else -- a result of e# (used for metaprogramming)
    : signal CompileError with "invalid form"
    }
  }
  in
: export Compile
}

As a matter of illustration, a compiled entity bound to the if keyword could be constructed by evaluating the expression

{ object {} with
  -- Classification
  IsRvalue = {proc {_} as False}
  IsLvalue = {proc {_} as False}
  -- Compilation
  Compile' =
  { proc { _; Form; SymTab } as
  : if (Size[Form] >= 6) & (Form[2] == then') & (Form[4] = else') then
    { let { _Cond; _Body1; _Body2 } in
    : object
      { _Cond  = CompileRvalue[Form[1]; SymTab]
        _Body1 = CompileRvalue[Form[3]; SymTab]
        _Body2 = CompileRvalues[Form.Elems[Range[5; Size[Form]]]; SymTab]
      }
      with
      -- Classification
      IsRvalue = {proc {_} as True}
      IsLvalue = {proc {Self} as Self[_Body1]@.IsLvalue[] & Self[_Body2]@.IsLvalue[]}
      -- Execution
      Execute = {proc {Self; Ctx} as Execute[Self[{if Execute[Self[_Cond]@; Ctx] then _Body1 else _Body2}]@; Ctx]}
      ExecIn  = {proc {Self; Val; Ctx} as ExecIn[Self[{if Execute[Self[_Cond]@; Ctx] then _Body1 else _Body2}]@; Val; Ctx]}
      ExecOut = {proc {Self; Ctx} as ExecOut[Self[{if Execute[Self[_Cond]@; Ctx] then _Body1 else _Body2}]@; Ctx]}
      -- Compilation
      Compile' = CompileApply
    }
    else
  : if (Size[Form] >= 4) & (Form[2] == then') then
    { let { _Cond; _Body } in
    : object
      { _Cond = CompileRvalue[Form[1]; SymTab]
        _Body = CompileRvalues[Form.Elems[Range[3; Size[Form]]]; SymTab]
      }
      with
      -- Classification
      IsRvalue = {proc {_} as True}
      IsLvalue = {proc {_} as False}
      -- Execution
      Execute = {proc {Self; Ctx} as: if Execute[Self[_Cond]@; Ctx] then Execute[Self[_Body]@; Ctx]}
      -- Compilation
      Compile' = CompileApply
    }
    else
  : signal CompileError with "invalid form"
  }
}

The above specification is very close to how expressions are evaluated in Lisp-family programming languages, although instead of evaluation we are talking here about compilation.

Conclusions

Please make your conclusions yourself about whether the presented above approach and the limitations of MANOOL make sense.


The End — Questions? — Email me: info@manool.org

Published:
Updated:
  1. With the same success, we could just view any existing Turing-complete programming language L as having only two operators — 0 and 10 appends the digit “0” to a buffer, whereas 1 appends “1”, and every eight digits the abstract machine feeds the corresponding octet to a translator of L, et voilà.

  2. For instance, according to these definitions, the syntax of Lisp-family programming languages (Common Lisp, Scheme, Clojure, Kernel, etc.) is considered to be exclusively the syntax of S-expressions (sometimes called the surface or concrete syntax), regardless of any further (structural) requirements placed on the corresponding Lisp data (i.e., the abstract syntax trees), since in practice, the later provide sufficient guidance to deduce the meaning of a phrase in Lisp by straightforward recursion (anyway).

  3. Strictly speaking, no Turing-complete language L has such limitations since we could always implement in the language L another Turing-complete language L' that would lack limitations from any chosen set of limitations, but I am talking here about cost-effective use cases only (i.e., in practical sense).

  4. For instance, at least one dialect of the programming language Scheme provides almost empty top-level binding environment by default. But MANOOL is not Scheme — although it has a similar architecture, it has at least a distinct syntax and a distinct data model, among other things.

  5. Some grammar formalisms distinct from Chomsky's grammars intrinsically support conflict-resolution policies, but I argue that this does not solve the problems we are talking about here.

  6. Other possible examples are Forth, Tcl, and other dialects of Lisp, but Kernel may be the most sophisticated of them. Besides, Forth is too low-level and Tcl leads to inevitably inefficient implementations.

  7. … such as checking of presence and placement of certain syntactic structures, (almost unavoidable) linear-time lookup of identifiers, etc. …

  8. They say that whoever does not understand Lisp is doomed to reinvent it … Still, MANOOL does differ from all existing Lisp-family programming languages in a number of important aspects.

  9. All examples in MANOOL presented in this article are complete, working examples that you can submit to the online evaluator to test.

  10. In practice, M, in turn, is a function of some context, such as a binding environment.

  11. The theory of syntactic analysis for formal languages has been developed quite well long ago, and for simplicity, such grammars may be limited to only one, synthesized attribute and only term constructors within attribute equations.

  12. I had to modify the MANOOL syntax only once in four years since its inception (due to the introduction of the \}...\{ notation for string literals).

  13. Yes, only 80%, or otherwise the syntax would become too complicated and overwhelmed with seldom used features (recall the Pareto principle!).

  14. I also noticed that due to relatively high regularity, complex constructs in MANOOL do not look disproportionally uglier than simple ones, as it happens with some programming language notations originally designed for “readability” and “elegance”.

  15. There is at least one such precedent in the Lisp family of languages — Clojure.

  16. While this looks like a contrived example, it is intended to illustrate various possibilities.

  17. This is not the first time that someone comes and suggests something to replace S-expressions and … fails (beginning from never implemented M-expressions from classic Lisp, which incidentally slightly resemble the notation of MANOOL). I do not, however, expect the same fate for MANOOL ;-).

  18. Understanding a metacircular specification for a language L requires prior knowledge of the language L, which raises a question about its utility. However, normally only partial knowledge is required, and this roughly corresponds to how children learn communicative systems (i.e., natural languages) from scratch.