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REDUCERON MEMO 9
F-lite: a core subset of Haskell
Matthew N, 26 November 2008
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F-lite is a core subset of Haskell.  Unlike GHC Core and Yhc Core,
F-lite has a friendly concrete syntax.  You can write F-lite programs
in a file, and pass them to the F-lite interpreter or compiler.
Another way to view F-lite is as a minimalist lazy functional
language.

F-lite is untyped
-----------------

But as it is a subset of Haskell, you can use a Haskell implementation
to type-check F-lite programs.  

EXAMPLE 0: F-lite definition of 'append'.  Definitions of 'Nil' and
'Cons' are not required - there is no need to define algebraic data
types.

  append Nil ys = ys;
  append (Cons x xs) ys = Cons x (append xs ys);

(The use of semi-colons to seperate equations is mandatory.)

F-lite supports uniform pattern matching
----------------------------------------

Pattern matching is uniform if and only if the order of equations
doesn't matter (Wadler '86).  Uniform pattern matching can be easily
and efficiently compiled to core case expressions.  A core case
expression is one whose patterns all have the form 'constructor
applied to zero or more variables'.  The fact that the order of
equations doesn't matter is also useful when transforming functional
programs, for example by fold/unfold transformations.

EXAMPLE 1: F-lite definition of 'zipWith', illustrating uniform
pattern matching.

  zipWith f Nil ys = Nil;
  zipWith f (Cons x xs) Nil = Nil;
  zipWith f (Cons x xs) (Cons y ys) = Cons (f x y) (zipWith f xs ys);

EXAMPLE 2: F-lite definition of 'init', illustrating nested,
incomplete, uniform pattern matching.

  init (Cons x Nil) = Nil;
  init (Cons x (Cons y ys)) = Cons x (init (Cons y ys));

EXAMPLE 3: F-lite definition of 'init', using a case expression.

  init xs = case xs of {
              Cons x Nil -> Nil;
              Cons x (Cons y ys) -> Cons x (init (Cons y ys));
            };

(The use of semi-colons to seperate case alternatives is mandatory.)

F-lite supports 'let'-expressions
---------------------------------

But they may only bind expressions to variables (not patterns).

EXAMPLE 4: F-lite definition of 'pow', the power-list function,
illustrating a let expression.

  pow Nil = Cons Nil Nil;
  pow (Cons x xs) = let { rest = pow xs } in
                      append rest (map (Cons x) rest);

EXAMPLE 5: F-lite definition of 'repeat', using a let expression to
introduce a cyclic data structure.

  repeat x = let { xs = Cons x xs } in xs;

F-lite supports primitive integers
----------------------------------

Finite precision integers along with the following arithmetic
functions are allowed: (+), (-), (<=), (==), (/=).  The latter three
return 'True' or 'False' accordingly.  These operators must be written
in prefix form and cannot be partially applied.

EXAMPLE 6: F-lite definition of 'negate'.

  negate n = (-) 0 n;

EXAMPLE 7:  F-lite definition of 'fromTo'.

  fromTo n m = case (<=) n m of {
                 True -> Cons n (fromTo ((+) n 1) m);
                 False -> Nil;
               };

F-lite supports printing
------------------------

Two primitives, 'emit' and 'emitInt', are provided for printing characters
and integers respectively.

EXAMPLE 8: Printing the string "hi!" in F-lite.

  sayHi k = emit 'h' (emit 'i' (emit '!' k))

When evaluated, 'sayHi k' will print "hi!" and return 'k' (the
continuation).

EXAMPLE 9: 'Hello world' in F-lite.

  emitStr Nil k = k;
  emitStr (Cons x xs) k = emit x (emitStr xs k);

  main = emitStr "Hello world!\n" 0;

String literals are internally translated to 'Nil'-'Cons' lists of
characters.  The result of the 'main' function is expected to be an
integer - the displaying of any output must be done explicitly by the
programmer.

EXAMPLE 10: Full F-lite program to display the 10th fibonacci number.

  {

  fib n = case (<=) n 1 of {
            True  -> 1;
            False -> (+) (fib ((-) n 2)) (fib ((-) n 1));
          };

  emitStr Nil k = k;
  emitStr (Cons x xs) k = emit x (emitStr xs k);

  main = emitStr "fib(10) = " (emitInt (fib 10) (emit '\n' 0));

}

The braces enclosing the program are indeed mandatory.  The primitive
'emitInt' function is like 'emit' but prints an integer rather than a
character.  Both 'emit' and 'emitInt' must be applied to at least one
argument.

Other syntactic sugar
---------------------

If-then-else expressions are allowed, and are desugared as follows.

  if e1 then e2 else e3    -->    case e1 of { True -> e2 ; False -> e3 }

Our implementation
------------------

Our F-lite implementation includes both an interpreter (written in
Haskell) and a compiler (to C code - see Memo 22).  It works in both
Hugs and GHC.  For example, in the source directory, using Hugs:

  > runhugs Flite.hs ../examples/Fib.hs
  fib(10) = 89

and likewise using GHC:

  > ghc -O2 --make Flite.hs -o Flite

  > ./Flite ../examples/Fib.hs
  fib(10) = 89

To compile F-lite programs, use the '--compile' command-line option,
and redirect the output to a C file of your choice.

  > ./Flite --compile ../examples/Fib.hs > /tmp/Fib.c

The resulting C file can be compiled (with optimisations) using GCC:

  > gcc -O3 /tmp/Fib.c -o Fib

  > ./Fib
  fib(10) = 89