swift/step2_eval.swift

   1 //******************************************************************************
   2 // MAL - step 2 - eval
   3 //******************************************************************************
   4 // This file is automatically generated from templates/step.swift. Rather than
   5 // editing it directly, it's probably better to edit templates/step.swift and
   6 // regenerate this file. Otherwise, your change might be lost if/when someone
   7 // else performs that process.
   8 //******************************************************************************
   9
  10 import Foundation
  11
  12 // Parse the string into an AST.
  13 //
  14 private func READ(str: String) throws -> MalVal {
  15     return try read_str(str)
  16 }
  17
  18 // Perform a simple evaluation of the `ast` object. If it's a symbol,
  19 // dereference it and return its value. If it's a collection, call EVAL on all
  20 // elements (or just the values, in the case of the hashmap). Otherwise, return
  21 // the object unchanged.
  22 //
  23 private func eval_ast(ast: MalVal, _ env: Environment) throws -> MalVal {
  24     if let symbol = as_symbolQ(ast) {
  25         guard let val = env.get(symbol) else {
  26             try throw_error("'\(symbol)' not found")    // Specific text needed to match MAL unit tests
  27         }
  28         return val
  29     }
  30     if let list = as_listQ(ast) {
  31         var result = [MalVal]()
  32         result.reserveCapacity(Int(list.count))
  33         for item in list {
  34             let eval = try EVAL(item, env)
  35             result.append(eval)
  36         }
  37         return make_list(result)
  38     }
  39     if let vec = as_vectorQ(ast) {
  40         var result = [MalVal]()
  41         result.reserveCapacity(Int(vec.count))
  42         for item in vec {
  43             let eval = try EVAL(item, env)
  44             result.append(eval)
  45         }
  46         return make_vector(result)
  47     }
  48     if let hash = as_hashmapQ(ast) {
  49         var result = [MalVal]()
  50         result.reserveCapacity(Int(hash.count) * 2)
  51         for (k, v) in hash {
  52             let new_v = try EVAL(v, env)
  53             result.append(k)
  54             result.append(new_v)
  55         }
  56         return make_hashmap(result)
  57     }
  58     return ast
  59 }
  60
  61 // Walk the AST and completely evaluate it, handling macro expansions, special
  62 // forms and function calls.
  63 //
  64 private func EVAL(ast: MalVal, _ env: Environment) throws -> MalVal {
  65
  66         if !is_list(ast) {
  67
  68             // Not a list -- just evaluate and return.
  69
  70             let answer = try eval_ast(ast, env)
  71             return answer
  72         }
  73
  74         // Special handling if it's a list.
  75
  76         let list = as_list(ast)
  77
  78         if list.isEmpty {
  79             return ast
  80         }
  81
  82         // Standard list to be applied. Evaluate all the elements first.
  83
  84         let eval = try eval_ast(ast, env)
  85
  86         // The result had better be a list and better be non-empty.
  87
  88         let eval_list = as_list(eval)
  89         if eval_list.isEmpty {
  90             return eval
  91         }
  92
  93         // Get the first element of the list and execute it.
  94
  95         let first = eval_list.first()
  96         let rest = as_sequence(eval_list.rest())
  97
  98         if let fn = as_builtinQ(first) {
  99             let answer = try fn.apply(rest)
 100             return answer
 101         }
 102
 103         // The first element wasn't a function to be executed. Return an
 104         // error saying so.
 105
 106         try throw_error("first list item does not evaluate to a function: \(first)")
 107 }
 108
 109 // Convert the value into a human-readable string for printing.
 110 //
 111 private func PRINT(exp: MalVal) -> String {
 112     return pr_str(exp, true)
 113 }
 114
 115 // Perform the READ and EVAL steps. Useful for when you don't care about the
 116 // printable result.
 117 //
 118 private func RE(text: String, _ env: Environment) -> MalVal? {
 119     if !text.isEmpty {
 120         do {
 121             let ast = try READ(text)
 122             do {
 123                 return try EVAL(ast, env)
 124             } catch let error as MalException {
 125                 print("Error evaluating input: \(error)")
 126             } catch {
 127                 print("Error evaluating input: \(error)")
 128             }
 129         } catch let error as MalException {
 130             print("Error parsing input: \(error)")
 131         } catch {
 132             print("Error parsing input: \(error)")
 133         }
 134     }
 135     return nil
 136 }
 137
 138 // Perform the full READ/EVAL/PRINT, returning a printable string.
 139 //
 140 private func REP(text: String, _ env: Environment) -> String? {
 141     let exp = RE(text, env)
 142     if exp == nil { return nil }
 143     return PRINT(exp!)
 144 }
 145
 146 // Perform the full REPL.
 147 //
 148 private func REPL(env: Environment) {
 149     while true {
 150         if let text = _readline("user> ") {
 151             if let output = REP(text, env) {
 152                 print("\(output)")
 153             }
 154         } else {
 155             print("")
 156             break
 157         }
 158     }
 159 }
 160
 161 func main() {
 162     let env = Environment(outer: nil)
 163
 164     load_history_file()
 165     load_builtins(env)
 166
 167     REPL(env)
 168
 169     save_history_file()
 170 }