ABC: A Better C

ABC was designed as a modern educational programming language, continuing the teaching philosophy of Pascal, but with the practical relevance of C.

Pascal was once an ideal language to learn programming from first principles. It allowed students to truly understand and implement fundamental concepts, such as:

• variables and types
• control flow and procedures
• memory organization
• stack and heap behavior

After learning Pascal, students were typically able to pick up other programming languages quickly and confidently, because they had developed a solid understanding of how software is fundamentally executed.

Today, many modern languages (such as Python) abstract away too many of these details, giving learners an incomplete mental model of what is really happening "under the hood." At the other extreme, languages like C expose all the low-level mechanisms — but suffer from syntactic pitfalls and semantic inconsistencies that make them unnecessarily difficult to learn, especially for beginners.

For example:

• complex declaration syntax (int *p[10] vs int (*p)[10])
• inconsistent treatment of arrays and structs in function calls
• subtle pointer behavior

ABC is intentionally designed to address these issues:

• The declaration syntax is inspired by Pascal, making it more readable and consistent.
• Semantic inconsistencies from C are deliberately cleaned up, thanks to the freedom of not requiring backwards compatibility.
• ABC offers a clear, consistent model of memory and data handling — allowing students to experience and understand the core problems that modern languages are designed to solve (e.g. memory management, ownership, aliasing).

Understanding these problems is critical:

You can only fully appreciate advanced solutions like Rust's Borrow Checker, reference counting, or garbage collection if you first understand the underlying challenges. ABC enables students to encounter these issues consciously, in a clean and accessible language.

In particular, ABC helps learners understand:

• global, local, and static variables
• stack vs. dynamic memory vs. global data
• parameter passing mechanisms
• memory layout of structs, arrays, and pointers
• explicit memory management
• how software is executed on the machine

ABC is therefore intended as a modern didactic programming language — not only for teaching how to program, but also for teaching how computers execute programs. It is suitable for use in:

• introductory programming courses
• systems programming courses
• high-performance computing courses
• compiler construction courses
• and as a foundation for understanding modern language features.

Installation

Verify that the correct LLVM version is used:

llvm-config --version
# should print 21.x (or higher)

Clone the repository, build, and install:

git clone https://github.com/michael-lehn/abc-llvm.git
cd abc-llvm
make
sudo make install

If needed, you can explicitly specify the C++ compiler and llvm-config when invoking make, for example:

make CXX=g++ llvm-config=llvm-config-21

Hints on Installing LLVM

ABC requires LLVM 21 (including llvm-config and clang).
Earlier LLVM versions will not work.

macOS (recommended):

brew install llvm@21

After installation, make sure the Homebrew LLVM tools are in your PATH:

export PATH="$(brew --prefix llvm@21)/bin:$PATH"

Debian/Ubuntu-based Linux:
Packages are available via the official LLVM APT repository:
https://apt.llvm.org

Arch Linux:

sudo pacman -S llvm clang

Fedora / RHEL / CentOS:

sudo dnf install llvm clang

Windows:

Windows is not supported natively.
Please install WSL (Windows Subsystem for Linux) and use the Linux instructions above.

Declarations in C and ABC

For comparison, these declarations in C:

    int *a[10];
    int (*b)[10];

are equivalent to these declarations in ABC:

    a: array[10] of -> int;
    b: -> array[10] of int;

In both cases, one declares

• a as an array of 10 elements, where each element is a pointer to an integer
• b as a pointer to an array of 10 integers

Occasionally, we need to specify that the data pointed to by a pointer should remain unchanged, or that the pointer itself should remain fixed, indicating that it shouldn't be redirected. In some cases, both conditions apply: neither the pointer nor the data it points to should change. Here are examples of such declarations in C:

    const int *c[10];
    int (* const d)[10];
    const int (* const e)[10];

It's important to note that in C, using const doesn't guarantee that the variable is immutable. With an appropriate cast, the content of a variable can still be altered. In ABC, the keyword readonly is used for this purpose. It signifies that while technically it's still possible to modify the value (if one really insists), the declaration clearly states the intent to access it in a read-only manner:

    c: array[10] of -> readonly int;
    d: array[10] of readonly -> int;
    e: array[10] of readonly -> readonly int;

Here, we have declared

• c as an array of 10 elements, where each element is a pointer to a readonly integer
• d as an array of 10 elements, where each element is a readonly pointer to an integer
• e as an array of 10 elements, where each element is a readonly pointer to a readonly integer

But that's not all about pointers. A function name represents an address, the address of its first instruction. Hence you can store the function address in a pointer variable. Such a pointer is then called a function pointer. Here, a declaration of a local or global pointer variable to a function that has no return type and does not accept any parameters:

void (*f)(void);

And here an example where f gets initialized such that it points to a function foo by simply assigning the function name:

void (*f)(void) = foo;

For function parameters, another syntax can be used which is more expressive. For example, here

void someFunction(void f(void));

function someFunction has a parameter f which is a function pointer with the same type declared above.

In ABC, you just have one way to declare such a function pointer:

f: -> fn();

Hence the declaration of someFunction becomes

fn someFunction(f: -> fn());

Let us consider some more exciting examples:

• With g: -> fn(:int) one declares a pointer to a function that has one parameter of type int and has no return type. Optionally you can use parameter names for readability which the compiler ignores. Hence g: -> fn(value :int) would be equivalent.
• With h: -> fn(:int):int or h: -> fn(value :int):int one declares a pointer to a function with one integer parameter and an integer return type.

Got the idea? Then you might already guess that

foo: -> fn(sel: int, value: int): -> fn(value: int): -> int;

declares a function pointer foo to a function with two integer parameters which returns a pointer to a function that has one integer parameter and returns a pointer to an integer. Now declare this in C without typedefs ;-)

Examples

@ <stdio.hdr>

fn main()
{
    printf("hello, world!\n");
}

The worst part of C is the C preprocessor (CPP). Hence it is greate that new C like languages are avoiding the preprocessor. But because ABC is just "A Better C, but still C" it does have a preprocessor. Students need to be prepared for this ugly side of C. However, compared to CPP the preprocessor has limited features. It can be used to include header files and you can define some simple macros.

Of course, the header file stdio.hdr does not contain the implementation of printf but just a declaration for it. From the preprocessor the compiler gets the following code:

extern fn printf(fmt: -> char, ...);

fn main()
{
    printf("hello, world!\n");
}

Compared to using CPP no include guards are required when the ABC preprocessor is used. Every file gets included only once (like using @pragma once with CPPs that support this pragma).

For teaching purposes (i.e. for showing the ugly side of C), consider this example:

@define X 42

fn main()
{
    local X: int = 42;
}

Here the ABC compiler receives the following code from the preprocessor:

fn main()
{
    local 42: int = 42;
}

Of course this triggers an error from the compiler. But it is hard to see from the error message the actual problem:

    local X: int = 42;
          ^^
macro.abc:5.11-5.12: error: expected local variable declaration list

Sure, the error message actually could show the code the compiler got from the preprocessor. But using the preprocessor should not be attractive. If you want to use symbols for literals use languages features, e.g. enum constants or constant expressions. Intead of macro functions use inline functions. Don't use a preprocessor.

Language Description

Lexical Elements

Comments can be delimited by /* and */. Nested comments are not supported. Alternatively, comments start with // and are ended by the next line terminator. Comments are treated like space characters.

Each program source is converted into a sequence of tokens during lexical analysis. Tokens can be punctuators consisting of one or more special characters, reserved keywords, identifiers, or literals. Tokens end if the next character is a space or if the next character cannot be added to it.

Punctuators are:

. ... ; : , { } ( ) [ ] ^ + ++ += - -- -= -> * *= / /= % %= = == ! != > >= < <= & && | || ? #

The following keywords are reserved and cannot be used as identifiers:

alignas alignof array break case const continue default do else enum extern fn for global goto if label local nullptr of return sizeof struct switch then type union while

Identifiers begin with a letter, i.e., A to Z and a to z, or an underscore _, and are optionally followed by a sequence of more letters, underscores, or decimal digits 0 to 9. Some identifiers are predefined.

The following predefined identifiers are used as named types (essentially keywords):

void bool u8 u16 u32 u64 i8 i16 i32 i64 int long long_long unsigned unsigned_long unsigned_long_long size_t ptrdiff_t float double

The following predefined identifiers are used as named constants:

nullptr

Literals can be decimal literals, octal literals, hexadecimal literals, string literals, character literals, and (decimal) floating point literals.

• Decimal literals begin with a digit 1 to 9 and optionally have more digits from 0 to 9. Decimal constants are unsigned and can be of arbitrary size.
• Octal literals begin with a digit 0 and optionally have more digits from 0 to 7.
• Hexadecimal literals begin with a prefix 0x and one or more digits from 0 to 9, 'a' to 'f', or 'A' to 'F'.
• String literals are delimited by ". Backslashes, i.e., \, are escape characters, removing the special meaning of the following character or allowing the insertion of special characters into a string.
• Character literals are delimited by ' and consist of a single character (which can be an escaped character).
• Currently only decimal (but not hexadecimal) floating point literals are supported (see floating_literal)

Expressions

The syntax for expressions is very similar to expressions in C, with the following exceptions:

• Expression lists are currently not implemented.
• Not all operators are currently supported. On the to-do list are bitwise operators, the alignas operator, and the alignof operator.
• For a conditional expression, the ternary Operator from C can be used, e.g., x = a > b ? y : z, or alternatively, the more verbose notation x = a > b then y else z. At least currently, "?" and "then", and ":" and "else" are interchangeable. That means x = a > b ? y else z and x = a > b then y : z are also alternatives.
• For type casts, the C syntax is not supported. Instead, the C++ style notation x = int(y) is used for casting the expression y to type int.
• Pointers can be dereferenced like in C with the prefix operator *. Additionally, the arrow operator -> can be used; i.e., the use of the operator -> is not restricted to "struct pointers". Hence, if x is a pointer to int, the expressions *x = 42 and x-> = 42 are equivalent, and in both cases, 42 is assigned to the integer at the end of pointer x.

The EBNF grammar for expressions is:

          expression-list = assignment-expression { "," assignment-expression}
    assignment-expression = conditional-expression { ("=" | "+=" | "-=" | "*=" | "/=" | "%=") assignment-expression }
   conditional-expression = logical-or-expression
                                [ ("?" | "then") assignment-expression
                                  (":" | "else") conditional-expression ]
    logical-or-expression = logical-and-expression [ "||" logical-and-expression ]
   logical-and-expression = equality-expression [ "&&" equality-expression ]
      equality-expression = relational-expression [ ("==" | "!=") relational-expression ]
    relational-expression = additive-expression [ ("<" | "<=" | ">" | ">=" ) additive-expression ]
      additive-expression = multiplicative-expression [ ("+" | "-" ) multiplicative-expression ]
multiplicative-expression = unary-prefix-expression [ ("*" | "/" | "%" ) unary-prefix-expression ]
  unary-prefix-expression = ("-" | "!" | "++" | "--" | "*" | "&") unary-prefix-expression
                          | postfix-expression
       postfix-expression = primary-expression
                          | postfix-expression "." identifier
                          | postfix-expression "->" [ identifier ]
                          | postfix-expression "[" expression-list "]"
                          | postfix-expression "(" expression-list ")" 
                          | postfix-expression "++" 
                          | postfix-expression "--"
       primary-expression = identifier
                          | "sizeof" "(" (type | expression-list) ")"
                          | "nullptr"
                          | decimal-literal
                          | octal-literal
                          | hexadecimal-literal

            compound-expression = string-literal
                                | "{" compound-expression-initializer { "," compound-expression-initializer } "}"
compound-expression-initializer = [designator "=" ] assignment-expression
                     designator = "." identifier"
                                | "[" assignment-expression "]" 

For convenience, the precedence and associativity are summarized in the following table:

Precedence	Associativity	Operators	Meaning
16 (highest)	left	Identifier Literal ++ (post-increment) -- (post-decrement) f() (function call) [i] (index operator) -> (indirect member access or dereference operator) s.member (direct member access)	Primary and Unary postfix expression
15	right	* (dereference operator) & (address operator) - (unary minus) + (unary plus) ! (logical not) ++ (pre-increment) -- (pre-decrement) sizeof type(expression)	Unary prefix expression
13	left	* (multiply) / (divide) % (modulo)	Multiplicative expression
12	left	+ (add) - (subtract)	Additive expression
10	left	< (less) > (greater) <= (less equal) >= (greater equal)	Relational expression
9	left	== (equal) != (not equal)	Equality and inequality expression
5	left	&&	Logical and
4	left	||	Logical or
3	right	? in conjunction with : or then in conjunction with else	Conditional expression
2	right	= += -= *= /= %=	Assignment

Types

                 type = [const] unqualified-type
     unqualified-type = named-type
                      | pointer-type
                      | array-type
                      | function-type
           named-type = identifier
         pointer-type = "->" type
           array-type = "array" array-dim-and-type
   array-dim-and-type = "[" assignment-expression "]" { "[" assignment-expression "]" } "of" type
        function-type = "fn" [identifier] "(" function-parameter-list ")" [ ":" type ]

Structure of an ABC Program

input-sequence        = {top-level-declaration} EOI
top-level-declaration = function-declaration-or-definition
                      | extern-declaration
                      | global-variable-definition
                      | type-declaration
                      | enum-declaration
                      | struct-declaration

Function Declarations and Definitions

function-declaration-or-definition = function-header (";" | function-body)
                   function-header = "fn" identifier "(" function-parameter-list ")" [ ":" type ]
           function-parameter-list = [ [identifier] ":" type { "," [identifier] ":" type} } ["," "..."] ]
                     function-body = compound-statement

         extern-declaration = "extern" ( function-declaration | extern-variable-declaration ) ";"
       function-declaration = function-type
extern-variable-declaration = identifier-list ":" type { "," identifier-list ":" type }
            identifier-list = identifier { "," identifier }

Global Variable Declarations and Definitions

global-variable-definition = "global" variable-definition-list ";"
  variable-definition-list = variable-definition { "," variable-definition }
       variable-definition = identifier-list ":" type
                                [ "=" initializer-expression ]
    initializer-expression = compound-expression
                           | assignment-expression

Type Aliases

type-declaration = "type" identifier ":" type ";"

Structured Type Declaration

       struct-declaration = "struct" identifier (";" | struct-member-declaration )
struct-member-declaration = "{" { ( "union" "{" struct-member-list "}"| struct-member-list) } "}" ";"
       struct-member-list = identifier { "," identifier } ":" ( type | struct-declaration ) ";"

Enumeration Type Declaration and Enumeration Constants

  enum-declaration = "enum" identifier ":" integer-type "{" { enum-constant-list } "}" ";"
enum-constant-list = identifier [ "=" assignment-expression] { "," identifier [ "=" assignment-expression] }

Statements

Compound Statements

            compound-statement = "{" { statement-or-declaration-list } "}"
 statement-or-declaration-list = "{" { statement | declaration } "}"
                     statement = compound-statement
                               | if-statement
                               | switch-statement
                               | while-statement
                               | do-while-statement
                               | for-statement
                               | return-statement
                               | break-statement
                               | continue-statement
                               | goto-statement
                               | label-definition
                               | expression-statement
                   declaration = type-declaration
                               | enum-declaration
                               | struct-declaration
                               | static-variable-definition
                               | local-variable-definition
    static-variable-definition = "static" variable-definition-list ";"
     local-variable-definition = "local" variable-definition-list ";"

Expression Statements

expression-statement = [expression-list] ";"

Control Structures

If-then-(else) statements

if-statement      = "if" "(" expression-list ")" compound-statement
                     [ "else" if-statement | compound-statement ]

Switch statements

        switch-statement = "switch" "(" expression-list ")" "{" switch-case-or-statement "}"
switch-case-or-statement = "case" expression-list ":"
                         | "default" ":"
                         | statement

Loops

While Loops

while-statement = "while" "(" expression-list ")" compound-statement

Do-While Loops

do-while-statement = do compound-statement "while" "(" expression-list ")" ";"

For Loops

for-statement = "for" "(" [expression-or-local-variable-definition]
                          [expression-list] ";" [expression-list] ")"
                          compound-statement
expression-or-local-variable-definition = expression-list ";"
                                        | local-variable-definition

Break and continue

return-statement = "return" [ expression-list ] ";"

break-statement = "break" ";"
continue-statement = "continue" ";"

Goto and labels

  goto-statement = "goto" identifier ";"
label-definition = "label" identifier ":"