Emin Arslan
8d301a6fc2
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105 lines
3.3 KiB
OCaml
105 lines
3.3 KiB
OCaml
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module SymbolTable = Map.Make(String);;
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let ( let* ) = Result.bind
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let traverse = Util.traverse
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(* literals are not modified. *)
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type literal = Core_ast.literal
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(* Note:
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all symbol accesses are replaced with either a local or global access.
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Local accesses a symbol in the local scope.
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Global accesses a symbol in the global scope.
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Lambda expressions are stripped of the symbol name of their single parameter.
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This name is not needed at runtime, as all symbol accesses will be resolved
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into an index into either the local scope linked list or the global symbol table.
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Set is also split into its global and local versions, just like Var.
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The rest aren't modified at all.
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*)
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type expression =
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| Literal of literal
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| Local of int
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| Global of int
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| Apply of expression * expression
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| Lambda of expression
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| If of expression * expression * expression
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| SetLocal of int * expression
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| SetGlobal of int * expression
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| Begin of expression list
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(* extract all defined global symbols, given the top-level expressions
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and definitions of a program
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The returned table maps symbol names to unique integers, representing
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an index into a global array where the values of all global symbols will
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be kept at runtime.
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*)
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let extract_globals (top : Core_ast.top_level list) =
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let id_counter = (ref (-1)) in
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let id () =
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id_counter := !id_counter + 1; !id_counter in
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let rec aux tbl = function
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| [] -> tbl
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| Core_ast.Define (sym, _) :: rest ->
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aux (SymbolTable.add sym (id ()) tbl) rest
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| Expr _ :: rest ->
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aux tbl rest
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in aux SymbolTable.empty top
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(* The current lexical scope is simply a linked list of entries,
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and each symbol access will be resolved as an access to an index
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in this linked list. The symbol names are erased before runtime.
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During this analysis we keep the lexical scope as a linked list of
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symbols, and we find the index by traversing this linked list.
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*)
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let resolve_global tbl sym =
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match SymbolTable.find_opt sym tbl with
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| Some x -> Ok (Global x)
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| None -> Error ("symbol " ^ sym ^ " is not defined!")
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let resolve_lexical tbl env sym =
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let rec aux counter = function
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| [] -> resolve_global tbl sym
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| x :: _ when String.equal x sym -> Ok (Local counter)
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| _ :: rest -> aux (counter + 1) rest
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in aux 0 env
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let resolve_symbol tbl env sym =
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resolve_lexical tbl env sym
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let resolve_set tbl env sym expr =
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let* sym = resolve_symbol tbl env sym in
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match sym with
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| Local i -> Ok (SetLocal (i, expr))
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| Global i -> Ok (SetGlobal (i, expr))
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| _ -> Error "resolve_set: symbol resolution returned something invalid."
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let rec analyze tbl current = function
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| Core_ast.Literal s -> Ok (Literal s)
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| Var sym -> resolve_symbol tbl current sym
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| Set (sym, expr) ->
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let* inner = analyze tbl current expr in
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resolve_set tbl current sym inner
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| Lambda (s, body) ->
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let* body = (analyze tbl (s :: current) body) in
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Ok (Lambda body)
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| Apply (f, e) ->
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let* f = analyze tbl current f in
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let* e = analyze tbl current e in
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Ok (Apply (f, e))
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| If (test, pos, neg) ->
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let* test = analyze tbl current test in
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let* pos = analyze tbl current pos in
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let* neg = analyze tbl current neg in
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Ok (If (test, pos, neg))
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| Begin el ->
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let* body = traverse (analyze tbl current) el in
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Ok (Begin body)
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