Evil ML

Evil ML is a joke compiler from ML to C++ template language (not ordinary C++ code).

C++ template is a higher-order pure functional programming language traditionally used for compile-time computation, while its syntax is verbose and hard to use. ML, a higher-order functional programming language, is simple, practical and easy to understand, so that we jokingly implemented this compiler. You can easily use black magic in C++ template programming.

P.S. constexpr (supported C++11 or above) is useful. Why don't you use it?

Features

• OCaml-like higher-order pure functional language (Hindley-Milner polymorphism, no value restriction).
• Type inference is performed. Most type annotations are automatically inferred.
• Variant, a flexible and strong data structure, is supported.
• You can write raw C++ code in (*! ... *) in top level.

Difference from OCaml:

• Strings have type char list (type string does not exist).
• Module system and separate compilation are not supported.
• User-defined operators are not allowed.
• type keyword in top level can only define variant types. You cannot declare aliases of types and records.
• Pattern match is only performed by match. Patterns cannot appear in formal arguments and l.h.s. of let bindings.
• Exhaustivity checking of pattern matching is not implemented. (future work)
• Identifiers are defined as regular expression [a-zA-Z_][a-zA-Z0-9_]*. Primes cannot be used, and names that begin __ml_ are reserved by this compiler. Identifiers of data constructors begin capital letters.
• Top-level shadowing of identifiers (variables, types, and constructors) is prohibited.

Demo: quick sort

examples/quicksort/qsort.ml implements quick sort of a list of 8 elements. You can compile the ML program into C++ template as online demo.

1. Check the check box of "Generate stand-alone code (embedding evilml.hpp)"
2. Push the button "Compile"
3. Copy and paste the generated C++ code into file qsort.cpp
4. Try to compile and run it:

$ g++ qsort.cpp
$ ./a.out
1  2  3  4  5  6  7  8

In order to make sure that sorting is executed in compile time, we suggest to use g++ -S qsort.cpp and open qsort.s:

...
	movl	$1, 4(%esp)   ; pass 1 to printf
	movl	$.LC0, (%esp)
	call	printf
	movl	$2, 4(%esp)   ; pass 2 to printf
	movl	$.LC0, (%esp)
	call	printf
	movl	$3, 4(%esp)   ; pass 3 to printf
	movl	$.LC0, (%esp)
	call	printf
	movl	$4, 4(%esp)   ; pass 4 to printf
	movl	$.LC0, (%esp)
	call	printf
	movl	$5, 4(%esp)   ; pass 5 to printf
	movl	$.LC0, (%esp)
	call	printf
	movl	$6, 4(%esp)   ; pass 6 to printf
	movl	$.LC0, (%esp)
	call	printf
	movl	$7, 4(%esp)   ; pass 7 to printf
	movl	$.LC0, (%esp)
	call	printf
	movl	$8, 4(%esp)   ; pass 8 to printf
	movl	$.LC1, (%esp)
	call	printf
...

(Of course, you can use std::cout to print integers in qsort.cpp, however we make use of printf for readable assembly code.)

Bugs

• let rec diverge _ = diverge () should be infinite loop, but generated C++ code causes compilation error. let rec diverge n = diverge (n+1) passes C++ compilation. (I don't know the formal definition of reduction rules of C++ template expressions.)
• C++03 template prohibits operation of float-point values, so that this compiler outputs wrong code.