title=The Generic Syntax Extension
authors=Çagdas Bozman
date=2014-04-01
category=OCaml
tag=tooling,ocaml,generic,syntax,extension

OCaml 4.01 with its new feature to disambiguate constructors allows to do a nice trick: a simple and generic syntax extension that allows to define your own syntax without having to write complicated parsetree transformers. We propose an implementation in the form of a ppx rewriter.

it does only a simple transformation: replace strings prefixed by an operator starting with ! by a series of constructor applications

for instance:

```ocaml
!! "hello 3"
```

is rewriten to

```ocaml
!! (Start (H (E (L (L (O (Space (N3 (End))))))))
```

How is that generic ? We will present you a few examples.

#### Base 3 Numbers
For instance, if you want to declare base 3 arbitrary big numbers, let's define a syntax for it. We first start by declaring some types.

```ocaml
type start = Start of p

and p =
  | N0 of stop
  | N1 of q
  | N2 of q

and q =
  | N0 of q
  | N1 of q
  | N2 of q
  | Underscore of q
  | End

and stop = End
```

This type will only allow to write strings matching the regexp 0 | (1|2)(0|1|2|_)*. Notice that some constructors appear in multiple types like N0. This is not a problem since constructor desambiguation will choose for us the right one at the right place. Let's now define a few functions to use it:

```ocaml
open Num

let rec convert_p = function
  | N0 (End) -> Int 0
  | N1 t -> convert_q (Int 1) t
  | N2 t -> convert_q (Int 2) t

and convert_q acc = function
  | N0 t -> convert_q (acc */ Int 3) t
  | N1 t -> convert_q (Int 1 +/ acc */ Int 3) t
  | N2 t -> convert_q (Int 2 +/ acc */ Int 3) t
  | Underscore t -> convert_q acc t
  | End -> acc

let convert (Start p) = convert_p p
```

```ocaml
# val convert : start -> Num.num = <fun>
```

And we can now try it:

```ocaml
let n1 = convert (Start (N0 End))
# val n1 : Num.num = <num 0>
let n2 = convert (Start (N1 (Underscore (N0 End))))
# val n2 : Num.num = <num 3>
let n3 = convert (Start (N1 (N2 (N0 End))))
# val n3 : Num.num = <num 15>
```

And the generic syntax extension allows us to write:

```ocaml
let ( !! ) = convert

let n4 = !! "120_121_000"
val n4 : Num.num = <num 11367>
```

#### Specialised Format Strings
We can implement specialised format strings for a particular usage. Here, for concision we will restrict to a very small subset of the classical format: the characters %, i, c and space

Let's define the constructors.

```ocaml
type 'a start = Start of 'a a

and 'a a =
  | Percent : 'a f -> 'a a
  | I : 'a a -> 'a a
  | C : 'a a -> 'a a
  | Space : 'a a -> 'a a
  | End : unit a

and 'a f =
  | I : 'a a -> (int -> 'a) f
  | C : 'a a -> (char -> 'a) f
  | Percent : 'a a -> 'a f
```

Let's look at the inferred type for some examples:

```ocaml
let (!*) x = x

let v = !* "%i %c";;
# val v : (int -> char -> unit) start = Start (Percent (I (Space (Percent (C End)))))
let v = !* "ici";;
# val v : unit start = Start (I (C (I End)))
```

This is effectively the types we would like for a format string looking like that. To use it we can define a simple printer:

```ocaml
let rec print (Start cons) =
  main cons

and main : type t. t a -> t = function
  | I r ->
    print_string "i";
    main r
  | C r ->
    print_string "c";
    main r
  | Space r ->
    print_string " ";
    main r
  | End -> ()
  | Percent f ->
    format f

and format : type t. t f -> t = function
  | I r ->
    fun i ->
      print_int i;
      main r
  | C r ->
    fun c ->
      print_char c;
      main r
  | Percent r ->
    print_string "%";
    main r

let (!!) cons = print cons
```

And voila!

```ocaml
let s = !! "%i %c" 1 'c';;
# 1 c
```

### How generic is it really ?
It may not look like it, but we can do almost any syntax we might want this way. For instance we can do any regular language. To explain how we transform a regular language to a type definition, we will use as an example the language a(a|)b

```ocaml
type start = Start of a

and a =
  | A of a';

and a' =
  | A of b
  | B of stop

and b = B of stop

and stop = End
```

We can try a few things on it:

```ocaml
let v = Start (A (A (B End)))
# val v : start = Start (A (A (B End)))

let v = Start (A (B End))
# val v : start = Start (A (B End))

let v = Start (B End)
# Characters 15-16:
#   let v = Start (B End);;
#                  ^
# Error: The variant type a has no constructor B

let v = Start (A (A (A (B End))))
# Characters 21-22:
#  let v = Start (A (A (A (B End))));;
#                        ^
# Error: The variant type b has no constructor A
```

Assumes the language is given as an automaton that:

- has 4 states, a, a', b and stop
- with initial state a
- with final state stop
- with transitions: a - A -> a' a' - A -> b a' - B -> stop b - B -> stop
let's write {c} for the constructor corresponding to the character c and

[c][/c]

for the type corresponding to a state of the automaton.

- For each state q we have a type declaration [q]
- For each letter a of the alphabet we have a constructor {a}
- For each transition p - l -> q we have a constructor {l} with parameter [q] in type [p]:

```ocaml
type [p] = {l} of [q]
```

- The End constructor without any parameter must be present in any final state
- The initial state e is declared by

```ocaml
type start = Start of [e]
```

### Yet more generic
In fact we can encode deterministic context free languages (DCFL) also. To do that we encode pushdown automatons. Here we will only give a small example: the language of well parenthesized words

```ocaml
type empty
type 'a r = Dummy

type _ q =
  | End : empty q
  | Rparen : 'a q -> 'a r q
  | Lparen : 'a r q -> 'a q

type start = Start of empty q

let !! x = x

let m = ! ""
let m = ! "()"
let m = ! "((())())()"
```

To encode the stack, we use the type parameters: Lparen pushes an r to the stack, Rparen consumes it and End checks that the stack is effectively empty.

There are a few more tricks needed to encode tests on the top value in the stack, and a conversion of a grammar to Greibach normal form to allow this encoding.

### We can go even further
#### a^n b^n c^n
In fact we don't need to restrict to DCFL, we can for instance encode the a^n.b^n.c^n language which is not context free:

```ocaml
type zero
type 'a s = Succ

type (_,_) p =
  | End : (zero,zero) p
  | A : ('b s, 'c s) p -> ('b, 'c) p
  | B : ('b, 'c s) q -> ('b s, 'c s) p

and (_,_) q =
  | B : ('b, 'c) q -> ('b s, 'c) q
  | C : 'c r -> (zero, 'c s) q

and _ r =
  | End : zero r
  | C : 'c r -> 'c s r

type start = Start of (zero,zero) p

let v = Start (A (B (C End)))
let v = Start (A (A (B (B (C (C End))))))
```

#### Non recursive languages
We can also encode solutions of Post Correspondance Problems (PCP), which are not recursive languages:

Suppose we have two alphabets A = { X, Y, Z } et O = { a, b } and two morphisms m1 and m2 from A to O* defined as

- m1(X) = a, m1(Y) = ab, m1(Z) = bba
- m2(X) = baa, m2(Y) = aa, m2(Z) = bb

Solutions of this instance of PCP are words such that their images by m1 and m2 are equal. for instance ZYZX is a solution: both images are bbaabbbaa. The language of solution can be represented by this type declaration:

```ocaml
type empty
type 'a a = Dummy
type 'a b = Dummy

type (_,_) z =
  | X : ('t1, 't2) s -> ('t1 a, 't2 b a a) z
  | Y : ('t1, 't2) s -> ('t1 a b, 't2 a a) z
  | Z : ('t1, 't2) s -> ('t1 b b a, 't2 b b) z

and (_,_) s =
  | End : (empty,empty) s
  | X : ('t1, 't2) s -> ('t1 a, 't2 b a a) s
  | Y : ('t1, 't2) s -> ('t1 a b, 't2 a a) s
  | Z : ('t1, 't2) s -> ('t1 b b a, 't2 b b) s

type start = Start : ('a, 'a) z -> start

let v = X (Z (Y (Z End)))
let r = Start (X (Z (Y (Z End))))
```

### Open question
Can every context free language (not deterministic) be represented like that ? Notice that the classical example of the palindrome can be represented (proof let to the reader).

### Conclusion
So we have a nice extension available that allows you to define a new syntax by merely declaring a type. The code is available on [github](https://github.com/chambart/generic_ppx). We are waiting for the nice syntax you will invent !

PS: Their may remain a small problem... If inadvertently you mistype something you may find some quite complicated type errors attacking you like a pyranha instead of a syntax error.