Molten C

Molten C – evolved C programming language #

Systems programming language that attempts to evolve the C language, exploring data-oriented evolution path for C.

Language allows seamless interop with C libraries of any complexity, including the C standard library. Although very similar, the language is not compatible with C on the source code level, but C libraries can be imported and used directly.

Clean and refined syntax #

Minimal changes to the C syntax, “modern” conveniences like optional commas in newline-separated lists, optional parentheses around conditionals and optional semicolons. At the same time, clearly misleading or ambiguous constructs are disallowed.

Int fact(Int n) {
  if n == 0 {
    return 1

  return n * fact(n - 1)

print(fact(5)) // 120

Module system and C integration #

Explicit imports and exports prevent global namespace name collisions and allow for faster incremental compilation. Each Molten C file is a module and isolated namespace. Build system that discovers and links all files and libraries together is built-in into compiler and requires no configuration.

C header files can be seamlessly imported, with no need to create wrapper libraries or “bindings”.

import "stdio.h" {printf}

export Int main() {
  printf("Hello, World!\n") // Hello, World!
  return 0

Expressive and safe #

Optional types allow null to be assigned to variables of any type and allow compiler to diagnose unchecked usage of null at compile time.

Bool hasPositive(Int ?x, Int ?y) {
  return ?x && x > 0 || ?y && y > 0

print(hasPositive(1, 1))     // true
print(hasPositive(1, null))  // true
print(hasPositive(null, 1))  // true
print(hasPositive(null, -1)) // false

Extended enumeration type works like a tagged union, it can hold additional data that is embedded into cases. Pattern matching is supported and compiler can verify exhaustiveness of switch statements, where all possible cases must be handled.

enum Animal {
  elephant {Int weight}

Bool isLargeElephant(Animal a) {
  switch a {
  case .lion:
    return false
  case .elephant {weight}:
    return weight > 3000

print(isLargeElephant((.elephant){.weight = 12000})) // true
print(isLargeElephant((.elephant){.weight = 1000}))  // false
print(isLargeElephant(.lion))                        // false

Raw Pointers And Memory Access #

All features of the C language, including raw pointers, manual memory allocation, bitfields and unions.

import "stdlib.h" {malloc, free}
import "stdio.h" {puts}
import "string.h" {strdup}

export Int main() {
  var Byte *raw *raw values = malloc(sizeof(Byte *raw) * 2)
  values[0] = strdup("foo")
  values[1] = null

  for var auto p = values; *p != null; p++ {
    puts(*p) // "foo"
  return 0

Compatibility with C #

Molten C syntax is not a superset of C, but the compiler can import C headers and interoperate with C libraries. This is possible because Molten C can parse C code and convert C headers on the fly. Code in Molten C can be integrated into C projects one file at a time.

Applications #

This is a “non-object-oriented” language, much more aligned with the data-oriented design paradigm. There are no classes, interfaces or type behaviors in the language. It uses another approach, featuring fully structural type system with support for opaque types for data hiding.

The name Molten C can mean various things:

  • reforged C
  • language that is not SOLID
  • yet another language inspired by C

Molten C is not a general purpose program language, but it attempts to be really good for:

  • software with low latency or memory requirements
  • low level software like operating systems or hardware drivers
  • graphics software like renderers or games
  • WebAssembly programs that need to be small in size

Implementation #

Molten C compiler is written in C and depends on LLVM for code generation. This allows it to generate fast code and rich debug information.

To support excellent C interop, compiler includes separate parsers for C and Molten C languages, as well as C preprocessor. Both parsers generate the same AST nodes. This allows any C headers to be imported into Molten C, which means Molten C compiler can compile arbitrary C source code.

There is an additional intermediate IR, that is similar to LLVM IR, but more high level. In the future, code analysis and optimization passes can be added.

Yet another new language? #

Molten C attempts to preserve the “spirit” of C, evolving it into more expressive and safe C. Data-oriented design paradigm had a large influence on the design of the language.

Project Status #

Compiler has been in development since 2018. It went through multiple full and partial rewrites and a number of experiments.

Source code is not publicly available yet. A few important features and refinements are missing and need to be done before the initial release. Current open-source release estimate is late 2021.

Author #

Serge Sergii Iefremov (Serge)
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