LibLetLib

Static typing grinding your gears? Want to fail even faster? auto not automatic enough? LibLetLib is the answer that proudly defies some C++ best practises while upholding functional programming ones. LibLetLib provides the speed of Smalltalk and the security of C in a single tight package.

LibLetLib is a C++ library/framework/runtime to provide duck typed, functional programming while still being compatible with all other C++ code. LibLetLib works seamlessly with other C++ code so it can be used only where desired. Supports all C++ versions from C++98 to C++20 including limited freestanding support. C++11 or later is required for most advanced features.

Installation

LibLetLib is header-only. Copy the libletlib folder and #include libletlib/libletlib.hpp file.

LibLetLib pollutes the global namespace by default with a few macros.

• curry (Injects the empty lambda to force unique instantiation of a template)
• lambda (Lambda capture and conversion to function pointer infrastructure)
• subroutine (Lambda capture and conversion to function pointer infrastructure)
• Subroutine (Utility)
• function (Lambda capture and conversion to function pointer infrastructure)
• Function (Utility)
• type (Utility to define objects with proper inheritance)
• contains (Utility in conjuction with type)
• constructor (Utility in conjuction with contains)
• construct (Utility to call constructor)
• with (Utility in conjuction with construct)
• member (Utility in conjuction with type)
• match (Utility for pattern matching)
• against (Utility in conjuction with match)
• otherwise (Utility in conjuction with match)
• st (Utility, first argument of function)
• nd (Utility, second argument of function)
• rd (Utility, third argument of function)

These can be disabled with defining the macro LIBLETLIB_DISABLE_MACRO, although there are no guarantees that the underlying definitions of the macros will stay the same between versions.

Introduction

Library provides the var class and its const typedef let. var is designed to be mutable and can be reassigned, but the mutating operators, assignment, decrement and increment are not const and thus cannot be used on a const var (or let for short) object.

LibLetLib allows all C++ fundamental types, lists of var, functions, and even its own object types to be wrapped by a single object. Functions support variable capture and can be curried. Object types allow operator overloading and all members use message passing for dynamic dispatch. LibLetLib uses macros to provide a sleek code appearance, but a more robust (and less macro-y) approach is possible which requires some extra typing.

Wrapping and unwrapping

var can be assigned single arguments of any of the wrappable types implicitly. In C++11 and later a list can also created through an initializer list.

var is implicitly coercable to all non-object pointer types it can wrap. var has implicit conversions to std::string and other corresponding string objects for different character arrays. Being implicitly convertable means that var will usually match multiple types of overloaded functions in which case defining the desired type of var must be defined either by using the type conversion operator functions or casting (which triggers these functions).

let a = 1;
let b = 2.0;
let c = "3";
let d = []function(return "2";);
std::cout << std::pow(b.operator double(), d) << std::endl; // 4

Functions are automatically called without arguments and their return values converted to the required type. Here the std::pow function is using the std::pow(double, double) variant of the function due to b being converted to double. Thus LibLetLib will call d with no arguments, convert the resulting string "2" to double and provide it to the std::pow.

Type priority

From highest to lowest:

1. std::nullptr_t (Only C++11 and later)
2. void* (Also holds objects C++11 and later)
3. list (Actual type of var*)
4. function (Actual type of var(*)(var const&, var const&))
5. subroutine (Actual type of void(*)(var const&, var const&))
6. char32_t* (Only C++11 and later)
7. char16_t* (Only C++11 and later)
8. char8_t* (Only C++20 and later)
9. wchar_t*
10. char*
11. long double
12. double
13. float
14. unsigned long long (Only C++11 and later)
15. long long (Only C++11 and later)
16. unsigned long
17. long
18. unsigned int
19. int
20. unsigned short
21. short
22. char32_t (Only C++11 and later)
23. char16_t (Only C++11 and later)
24. char8_t (Only C++20 and later)
25. wchar_t
26. unsigned char
27. signed char
28. char
29. bool

Functions and Subroutines

LibLetLib functions are divided into two different categories: returning functions and non-returning subroutines. Functions can be defined as normal functions using the Function and Subroutine macros:

Function(foo) {
    // code
}

For C++11 and later, the lambda function macros enable more fluent expressions:

let foo = [/* capture */]function(/* code */);
let bar = [/* capture */]subroutine(/* code */);

and a convenience macro lambda for simple oneliner functions:

#define lambda(expression) []function(return expression;) 

The actual function signature for LibLetLib functions and subroutines is:

typedef var (*function) (var const& self, var const& args);
typedef void (*subroutine) (var const& self, var const& args);

The noexcept attribute is defined unless compiling for C++98 which doesn't support noexcept or using exceptions as the error handling mechanism (which is the default).

Arguments

Function and subroutine arguments are the const reference to self, which is the var reference to the function itself and args, which contains all the actual arguments as a list. This means that all functions and subroutines can take variable arguments and the arguments are anonymous. Three macros for the first three elements of args are provided for quality of life:

#define st (length(args) > 0 ? args[0] : var()) // 1st
#define nd (length(args) > 1 ? args[1] : var()) // 2nd
#define rd (length(args) > 2 ? args[2] : var()) // 3rd

Currying

LibLetLib functions can be curried with the macro function curry.

let fun = lambda(st + nd + rd);
let curried = curry(fun, 1, 1);
std::cout << curried(1) << std::endl; // 3

The invocation of the curry method requires an unique instantiation of a template function each time, which in turn involves a macro curry to provide an empty lambda to the curry method as a dummy. (C++ standard guarantees all lambdas are unique types and thus lead to unique instantiations of the template function.)

#define curry(...) libletlib::detail::curry_([](){}, __VA_ARGS__)

This is required so that the new curried lambda can remember the now enclosed function pointer as a static variable of the unique instantiation of curry. This could be made cleaner in C++20 which allows lambdas in unevaluated contexts, but broad compiler support isn't there yet. Due to this inconvenient required extra parameter to force unique instantiations the libletlib::detail::curry_ function itself cannot be curried, although every other LibLetLib utility function can be.

let SpicyRange = curry(range, 0, 100); // Generates ranges from 0 to 99 with still yet unprovided step.
let FullRange = SpicyRange(1); // 0 - 99, step of 1.
let HalfRange = SpicyRange(2); // 0 - 99, step of 2.

Pattern Matching

Rudimentary pattern matching for comparison between values and array/object layouts. Comparisons are to be given as predicate functions and results are computed by a function that is automatically given the arguments of the match(...) statement in the order they were given to it.

let foo = list(1, "2", lambda(3));
let bar = list(1, "3", lambda(2));

let result = match(foo, bar) against // result = [1, "2", function(return 3;)]
    | lambda(st != nd) ->* lambda(st) // This branch will be selected.
    | otherwise ->* lambda(nd);

let oof = 3;
let rab = 2.5;

let result2 = match(oof, rab) against // result2 = 3
	| lambda(st < nd) ->* lambda(nd)
	| otherwise ->* lambda(st); //This branch will be selected, because nd < st.
	
let lists = list(list(1, 2), list(1, 2, 3));

let result3 = match(lists) against
    | "#[(#[(i)])]" ->* var(true) //Match for list of lists of only integers.
    | otherwise ->* false;

Classes and Objects

LibLetLib objects have a three level inheritance structure.

┌─────────────────────────────┐ // MetaRoot is abstract and eventually all objects are 
│                             │ // cast to MetaRoot when performing operations on them.
│                             │ // 
│           MetaRoot          │ // MetaRoot has only one field: 'inner' which is a var of list type.  
│                             │ // This contains all the members.
└──────────────┬──────────────┘ 
               │                
               │
               │
┌──────────────▼──────────────┐ // Root implements the copy() and clone() methods that allow 
│                             │ // us to create additional instances of Inheritor.
│  template<class Inheritor>  │ //
│  Root : public MetaRoot     │ // This pattern is also known the 
│                             │ // Curiously Recurring Template Pattern (CRTP).
└──────────────┬──────────────┘
               │
               │
               │
┌──────────────▼──────────────┐ // Custom LibLetLib classes only contains the 
│                             │ // user-defined initialization for the MetaRoot 'inner' field that 
│                             │ // provides all the user-defined functionality.
│   Foo : public Root<Foo>    │ 
│                             │ 
└─────────────────────────────┘

Classes can be defined using macros or by hand. The declarations for class Foo below are syntactically identical:

type(Foo)
    contains(
		constructor(
			message("x") = 1;
			)
			
    	    member(msg) = "Hello World!"
    )

    	
class Foo final : public Root<Foo> {
	Foo() {
		inner = {
			{"Foo", [&]subroutine(this->message("x") = 1;)},
			{"msg", "Hello World!"},
		};
	}
};    	

Instantiation

Objects are instantiated through the new keyword, the var object they are assigned to will manage the memory according to RAII principles.

type(Foo)
    contains
    (
		constructor(
			message("onlyConstructed") = st;
			)
		
        member(func) = lambda(st + nd)
    )
    
int main(void)
{
    let foo = new Foo;
    std::cout << foo.message("func")(2, 2) << std::endl; // 4
	let foo2 = construct(Foo) with(true);
	std::cout << foo2.message("onlyConstructed") << std::endl; // true
    return 0;
}    

Member access

Object members can be accessed through the message(char const*) method or via the [] operator. Another way is to use the objectify() and then using MetaRoot method message(char const*).

type(Foo)
    contains
    (
		constructor()
        member(msg) = "Hello World!"
    )
    
int main(void) 
{
    let foo = new Foo;
    std::cout << foo.message("msg") << std::endl; // Returns msg by reference.
    std::cout << foo.objectify()->message("msg") << std::endl; // Returns msg by reference.
    
    std::cout << foo["msg"] << std::endl; // Returns msg by value.
}    

Operator overloading

Objects can define members with special names that will be used when the object is a part of a operator sequence. The operator overload of the left side of binary operators involving multiple objects is always used.

type(Foo)
    contains(
		constructor()
    	    member(+) = lambda(st.message(name) + nd.message(name))
    	)
    	
int main(void) {
	let x = new Foo;
	let y = new Foo;
	std::cout << x + y << std::endl; // "FooFoo"
	return 0;
}

Operator overloading member names

• foo@bar only the operator e.g. "+".
• @foo operator postfixed by "self", e.g. "+self"
• foo() ()
• foo[] []
• ++foo, foo++ ++ (postfix returns value before increment.)
• --foo, foo-- -- (postfix returns value before decrement.)

Error Handling

LibLetLib supports four models of error handling.

1. Exceptions via var. Enabled by default and the only one supported for freestanding.
2. Jump to handler via longjmp().
3. Error number style a la <cerrno>.
4. No error handling.

These are enabled by their respective macros:

1. LIBLETLIB_ERROR_EXCEPTION
2. LIBLETLIB_ERROR_LONGJMP
3. LIBLETLIB_ERROR_ERRNO
4. LIBLETLIB_ERROR_NONE

all of which undefine the other options when defined.

Exceptions

Exceptions thrown:

• std::domain_error Divide by zero, other logical errors, and forbidden casts.
• std::range_error Out of bounds access.
• std::runtime_error Indicates possible future support, but no functionality implemented.

Exceptions contain an error message describing the error.

Longjmp

Longjmp must be initialised before using LibLetLib with the LIBLETLIB_INIT(callback) macro where callback must be a LibLetLib subroutine or var(). The subroutine will be provided with an error number out of:

• LIBLETLIB_EDOM (Domain Error) Divide by zero, other logical errors, and forbidden casts.
• LIBLETLIB_ERANGE (Erraneous Range) Out of bounds access.
• LIBLETLIB_ENOTSUP (Not Supported, indicates possible future support.)

which assume any predefined value for <cerrno> macros EDOM, ERANGE, and ENOTSUP, but define their own if they are undefined. The default handler attempts to print the error into stderr and then attempts a graceful std::exit() with exit code corresponding to the error code.

Error Numbers

The errno macro defined by <cerrno> will be set to one of the corresponding values defined in <cerrno>:

• EDOM Divide by zero, other logical errors, and forbidden casts.
• ERANGE Out of bounds access.
• ENOTSUP (C++11 or later, indicates possible future support.)

The LibLetLib error code macros also resolve to the corresponding <cerrno> macro while LIBLETLIB_ERROR_ERRNO is enabled.

No Error Handling

LibLetLib silently ignores errors.

Freestanding

LibLetLib supports freestanding compilation with the following restrictions:

• No lists, functions, subroutines, strings of any type, or objects. (No heap allocation.)
• No wchar_t, char8_t, char16_t, char32_t support. (No <cwchar> and <cuchar> available.)
• Uses some self-implemented platform agnostic maths functions. (No <cmath> available.)

Which basically leaves all numeric types, standardchar character types, and bool for use.

C++98

LibLetLib supports C++98, but the lack of variadic templates and lambda functions limits the functionality:

• list() and function calls i.e. operator() have a 10 argument maximum. (Lists can still be grown arbitrarily programmatically.)
• No lambda functions, but regular function pointers can still be assigned to var and used normally.
• No currying, i.e. curry (Implementation requires lambda functions.)
• No pattern matching, i.e. match,with,otherwise. (Implementation requires variadic templates.)
• No objects, i.e. type. (Implementation requires variadic templates.)

More examples

Recurse you a factorial

#include "libletlib/libletlib.hpp"

using namespace libletlib;

let factorial = []function(
	    if(st <= 1)
	    	return 1;
	    return st * self(st - 1); // Because functions are anonymous
	);                            // they can call themselves with 'self'
	

int main(void) {
	std::cout << factorial(5) << std::endl;
}

(Im)mutability

#include "libletlib.hpp"
#include <cstdlib>
#include <iostream>

using namespace libletlib;

int main(void) {
    let x = 1;
    let y = 2;
    std::cout << x+y << std::endl; // 1 + 2 = 3, then output to std::cout.
    x += y; // Error, no += defined for let (const var).
    x = 2; // Error, no = defined for let (const var).
    var z = 2;
    z += y; // 2 + 2 = 4
    std::cout << z << std::endl; // Prints "4".
    return EXIT_SUCCESS;
}

A scenic route to say Hello World!

#include "libletlib/libletlib.hpp"
#include <cstdlib>
#include <iostream>

using namespace libletlib;

// A LibLetLib class with name of Foo.
type(Foo) contains(
	constructor()
	// Contains member + that overloads binary + operator for this class.
	// In LibLetLib all function arguments are provided in an array called args.
	// For convenience the first three arguments can be accessed through the macros 
	// st, nd, and rd (1st, 2nd, 3rd argument respectively).
	// the q operator queries an object for a member of given name.
	member(+) = lambda(st.message(msg) + " " + nd.message(msg);)
	
	// Member by the name of "message" and value of "Hello" on initialization of this class.	
	member(msg) = "Hello"	
	)

// PrintFirst is a subroutine (non-returning function) that simply prints the first argument given.	
let PrintFirst = []subroutine(std::cout << st << std::endl;);

int main(void) {
    // Create a pair of Foo instances.
    let foo1 = new Foo;
    let foo2 = new Foo;
    
    // Lets tweak the message on the second one to create our Hello World! message with the + operator.
    foo2.message("msg") = "World!";
    
    // Print our Hello World! (args is an array and thus surrounded by brackets.)
    PrintFirst(foo1 + foo2); // [Hello World!]
    
    return EXIT_SUCCESS;
}