core_ast: add initial draft for the core ast, and a conversion function from the syntactic ast

lib/compiler/core_ast.ml

@@ -0,0 +1,81 @@
+
+type literal =
+  | Int of int
+  | Double of float
+  | String of string
+  | Nil
+  | Cons of literal * literal
+
+(* The Core Abstract Syntax Tree.
+   This tree does not use a GADT, as every type of expression
+   will be reduced to its simplest equivalent form before ending
+   up here. There is no reason to make this tree typed.
+ *)
+type expression =
+  | Literal of literal
+  | Var of string
+  | Apply of expression * expression list
+  | Lambda of string list * expression
+  | Let of (string * expression) list * expression
+  | LetRec of (string * expression) list * expression
+  | If of expression * expression * expression
+  | Set of string * expression
+  | Begin of expression list
+
+
+type top_level =
+  | Define of string * expression
+  | Expr of expression
+
+
+
+
+
+let rec pair_of_def : Syntactic_ast.definition Syntactic_ast.t -> string * expression = function
+  | Syntactic_ast.Define (s, e) -> (s, of_expr e)
+and pair_of_binding = function
+  | Syntactic_ast.LetBinding (s, e) -> (s, of_expr e)
+and pair_of_clause = function
+  | Syntactic_ast.CondClause (e1, e2) -> (of_expr e1, of_expr e2)
+(* We convert a body into a regular letrec form.
+   A body is defined as a series of definitions followed by a series
+   of expressions. The definitions behave exactly as a letrec, so
+   it makes sense to convert the body into a normal letrec.
+ *)
+and of_body : Syntactic_ast.body Syntactic_ast.t -> expression = function
+  | Body (defs, exprs) ->
+     let exprs = List.map of_expr exprs in
+     let defs = List.map pair_of_def defs in
+     let b = Begin exprs in
+     LetRec (defs, b)
+
+(* TODO: currently this ignores the "optional" part of the lambda list,
+   fix this *)
+and of_ll : Syntactic_ast.lambda_list Syntactic_ast.t -> string list = function
+  | LambdaList (sl, _) -> sl
+
+and of_expr : Syntactic_ast.expression Syntactic_ast.t -> expression = function
+  | LitNil -> Literal Nil
+  | LitInt x -> Literal (Int x)
+  | LitDouble x -> Literal (Double x)
+  | LitString x -> Literal (String x)
+  | Var x -> Var x
+  | QuotedList _ -> failwith "TODO: rethink how quoted lists should work, the current definition makes no sense."
+  | Lambda (ll, b) -> Lambda (of_ll ll, of_body b)
+  | Let (bindings, b) -> Let (List.map pair_of_binding bindings, of_body b)
+  | LetRec (bindings, b) -> LetRec (List.map pair_of_binding bindings, of_body b)
+  | Cond (clauses) -> 
+     List.fold_right
+       (fun (e1, e2) acc -> If (e1, e2, acc))
+       (List.map pair_of_clause clauses)
+       (Literal Nil)
+  | If (e1, e2, e3) ->
+     If (of_expr e1, of_expr e2, of_expr e3)
+  | Set (s, e) -> Set (s, of_expr e)
+  | Apply (f, es) -> Apply (of_expr f, List.map of_expr es)
+
+
+and of_syntactic : Syntactic_ast.top_level -> top_level = function
+  | Def (Define (s, e)) -> Define (s, of_expr e)
+  | Exp (e) -> Expr (of_expr e)
+  | _ -> .