Evil ML

Evil ML is a joke compiler from ML to C++ template language (not ordinary C++ code). Please, don't use this for practical purposes.

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. This will give you nightmares.

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 types are automatically inferred.
Variant types are supported.
You can write raw C++ code in (*! ... *) in top level.
#use "foo.ml" loads .ml files in top level (double semi-colons ;; are not needed at the end). The .ml files you can load are found in directory evilml/include.

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.

Install

./configure
make
make install

Usage

You can compile foo.ml as follows:

evilml foo.ml

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.

Check the check box of "Generate stand-alone code (embedding evilml.hpp)"
Push the button "Compile"
Copy and paste the generated C++ code into file qsort.cpp
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.

dforgeas / evilml