argos/eval/eval.go

package eval

// TODO: a lot of code duplication that can be solved by a primitive registry
// TODO: a lot of code duplication that can be solved by a type checking helper

import (
  "fmt"

  "github.com/hellerve/argos/ast"
)

var trueVal = ast.AST{ast.Bool, true}
var falseVal = ast.AST{ast.Bool, false}
var nilVal = ast.AST{ast.List, []ast.AST{}}

type env struct {
  parent *env
  values map[string]*ast.AST
}

func newEnv(parent *env) env {
  return env{parent, make(map[string]*ast.AST)}
}

func ParentEnv() env {
  return newEnv(nil)
}

func (e env) Lookup(elem string) (*ast.AST, error) {
  res, ok := e.values[elem]

  if !ok {
    if e.parent == nil {
      return nil, fmt.Errorf("Symbol not found: %s", elem)
    }

    return e.parent.Lookup(elem)
  }

  return res, nil
}

func checkArity(input []ast.AST, arity int, name string) error {
  ilen := len(input)
  if ilen != arity+1 {
    return fmt.Errorf("Argument count to '%s' must be %d, was %d", name, arity, ilen)
  }

  return nil
}

func evalDef(input []ast.AST, e env) (*ast.AST, error) {
  err := checkArity(input, 2, "=")

  if err != nil {
    return nil, err
  }

  variable := &input[1]

  if variable.Tag != ast.Symbol {
    variable, err = Eval(variable, e)

    if variable.Tag != ast.Symbol {
      return nil, fmt.Errorf("First argument to 'def' must be symbol, was %v", variable.Tag)
    }
  }

  if err != nil {
    return nil, err
  }

  sym := variable.Val.(string)

  evald, err := Eval(&input[2], e)

  if err != nil {
    return nil, err
  }

  e.values[sym] = evald

  return evald, nil
}

func arithCast(input ast.AST, e env) (float64, error) {
  evald, err := Eval(&input, e)
  if err != nil {
    return 0, err
  }

  if evald.Tag != ast.Num {
    return 0, fmt.Errorf("Cannot perform arithmetic on ", evald.Pretty())
  }

  return evald.Val.(float64), nil
}

func evalArith(input []ast.AST, e env, fn (func(float64, float64) float64)) (*ast.AST, error) {
  ilen := len(input)
  if ilen < 3 {
    return nil, fmt.Errorf("Arithmetic functions take at least 2 arguments, got %d", ilen)
  }

  acc, err := arithCast(input[1], e)

  if err != nil {
    return nil, err
  }

  for _, elem := range input[2:] {
    val, err := arithCast(elem, e)

    if err != nil {
      return nil, err
    }
    acc = fn(acc, val)
  }

  res := ast.AST{ast.Num, acc}
  return &res, nil
}

func evalEq(input []ast.AST, e env) (*ast.AST, error) {
  err := checkArity(input, 2, "iso")

  if err != nil {
    return nil, err
  }

  x, err := Eval(&input[1], e)

  if err != nil {
    return nil, err
  }

  y, err := Eval(&input[2], e)

  if err != nil {
    return nil, err
  }

  if x.Tag != y.Tag {
    return &falseVal, nil
  }

  if x.Val == y.Val {
    return &trueVal, nil
  }
  return &falseVal, nil
}

func evalCons(input []ast.AST, e env) (*ast.AST, error) {
  err := checkArity(input, 2, "cons")

  if err != nil {
    return nil, err
  }

  lst, err := Eval(&input[2], e)

  if err != nil {
    return nil, err
  }

  if lst.Tag != ast.List {
    return nil, fmt.Errorf("Cannot cons to non-list %s", lst.Pretty())
  }

  fst, err := Eval(&input[1], e)

  if err != nil {
    return nil, err
  }

  res := ast.AST{ast.List, append([]ast.AST{*fst}, lst.Val.([]ast.AST)...)}

  return &res, nil
}

func evalCar(input []ast.AST, e env) (*ast.AST, error) {
  err := checkArity(input, 1, "car")

  if err != nil {
    return nil, err
  }

  lst, err := Eval(&input[1], e)

  if err != nil {
    return nil, err
  }

  if lst.Tag != ast.List {
    return nil, fmt.Errorf("Cannot car from non-list %s", lst.Pretty())
  }

  res := lst.Val.([]ast.AST)[0]
  return &res, nil
}

func evalCdr(input []ast.AST, e env) (*ast.AST, error) {
  err := checkArity(input, 1, "cdr")

  if err != nil {
    return nil, err
  }

  lst, err := Eval(&input[1], e)

  if err != nil {
    return nil, err
  }

  if lst.Tag != ast.List {
    return nil, fmt.Errorf("Cannot cdr from non-list %s", lst.Pretty())
  }

  res := ast.AST{ast.List, lst.Val.([]ast.AST)[1:]}
  return &res, nil
}

func evalNull(input []ast.AST, e env) (*ast.AST, error) {
  err := checkArity(input, 1, "null")

  if err != nil {
    return nil, err
  }

  lst, err := Eval(&input[1], e)

  if err != nil {
    return nil, err
  }

  if lst.Tag != ast.List {
    return nil, fmt.Errorf("Cannot call null? on non-list %s", lst.Pretty())
  }

  res := ast.AST{ast.Bool, len(lst.Val.([]ast.AST)) == 0}
  return &res, nil
}

func evalList(input *ast.AST, e env) (*ast.AST, error) {
  l := input.Val.([]ast.AST)
  head := l[0]

  if head.Tag != ast.Symbol {
    err := fmt.Errorf("Calling non-symbol: %s", head.Pretty())

    return nil, err
  }

  sym := head.Val.(string)

  switch sym {
    case "=":
      return evalDef(l, e)
    case "iso":
      return evalEq(l, e)
    case "quote":
      res := l[1]
      return &res, nil
    case "cons":
      return evalCons(l, e)
    case "car":
      return evalCar(l, e)
    case "cdr":
      return evalCdr(l, e)
    case "null?":
      return evalNull(l, e)
    case "+":
      return evalArith(l, e, func(x float64, y float64) float64 { return x + y })
    case "-":
      return evalArith(l, e, func(x float64, y float64) float64 { return x - y })
    case "*":
      return evalArith(l, e, func(x float64, y float64) float64 { return x * y })
    case "/":
      return evalArith(l, e, func(x float64, y float64) float64 { return x / y })
    case "%":
      return evalArith(l, e, func(x float64, y float64) float64 { return float64(int64(x) % int64(y)) })
  }
  return input, nil
}

func Eval(input *ast.AST, e env) (*ast.AST, error) {
  if (input.Tag == ast.List) {
    return evalList(input, e)
  }
  if (input.Tag == ast.Symbol) {
    val := input.Val.(string)

    var err error
    var res *ast.AST
    switch val {
      case "true":
        res = &trueVal
      case "false":
        res = &falseVal
      case "nil":
        res = &nilVal
      default:
        res, err = e.Lookup(val)
    }
    return res, err
  }
  return input, nil
}