serene-golang-implementation/bootstrap/pkg/core/eval.go

/*
 Serene --- Yet an other Lisp

Copyright (c) 2020  Sameer Rahmani <lxsameer@gnu.org>

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 2 of the License.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

package core

import (
	"serene-lang.org/bootstrap/pkg/ast"
)

// evalForm evaluates the given expression `form` by a slightly different
// evaluation rules. For example if `form` is a list instead of the formal
// evaluation of a list it will evaluate all the elements and return the
// evaluated list
func evalForm(rt *Runtime, scope IScope, form IExpr) (IExpr, IError) {

	switch form.GetType() {
	case ast.Nil:
		return form, nil
	case ast.Number:
		return form, nil

	// Symbol evaluation rules:
	// * If it's a NS qualified symbol (NSQS), Look it up in the external symbol table of
	// the current namespace.
	// * If it's not a NSQS Look up the name in the current scope.
	// * Otherwise throw an error
	case ast.Symbol:
		symbolName := form.(*Symbol).GetName()

		switch symbolName {
		case "true":
			return &True, nil
		case "false":
			return &False, nil
		case "nil":
			return &Nil, nil
		default:
			expr := scope.Lookup(symbolName)

			if expr == nil {
				return nil, MakeRuntimeErrorf(rt, "can't resolve symbol '%s' in ns '%s'", symbolName, rt.CurrentNS().GetName())
			}

			return expr.Value, nil
		}

	// Evaluate all the elements in the list instead of following the lisp convention
	case ast.List:
		var result []IExpr

		lst := form.(*List)

		for {
			if lst.Count() > 0 {
				expr, err := EvalForms(rt, scope, lst.First())
				if err != nil {
					return nil, err
				}
				result = append(result, expr)
				lst = lst.Rest().(*List)
			} else {
				break
			}
		}
		return MakeList(result), nil
	}

	// Default case
	return nil, MakeError(rt, "not implemented")

}

// EvalForms evaluates the given expr `expressions` (it can be a list, block, symbol or anything else)
// with the given runtime `rt` and the scope `scope`.
func EvalForms(rt *Runtime, scope IScope, expressions IExpr) (IExpr, IError) {
	// EvalForms is the main and the most important evaluation function on Serene.
	// It's a long loooooooooooong function. Why? Well, Because we don't want to
	// waste call stack spots in order to have a well organized code.
	// In order to avoid stackoverflows and implement TCO ( which is a must for
	// a functional language we need to avoid unnecessary calls and keep as much
	// as possible in a loop.
	var ret IExpr
	var err IError

tco:
	for {
		// The TCO loop is there to take advantage or the fact that
		// in order to call a function or a block we simply can change
		// the value of the `expressions` and `scope`
		var exprs []IExpr

		// Block evaluation rules:
		// * If empty, return Nothing
		// * Otherwise evaluate the expressions in the block one by one
		//   and return the last result
		if expressions.GetType() == ast.Block {
			if expressions.(*Block).Count() == 0 {
				return &Nothing, nil
			}
			exprs = expressions.(*Block).ToSlice()
		} else {
			exprs = []IExpr{expressions}
		}

	body:
		for _, forms := range exprs {
			// Evaluating forms one by one

			if forms.GetType() != ast.List {
				ret, err = evalForm(rt, scope, forms)
				break tco // return ret, err
			}

			list := forms.(*List)

			// Empty list evaluates to itself
			if list.Count() == 0 {
				ret = list
				break tco // return &Nil, nil
			}

			rawFirst := list.First()
			sform := ""

			// Handling special forms by looking up the first
			// element of the list. If it is a symbol, Grab
			// the name and check it for build it forms.
			//
			// Note: If we don't care about recursion in any
			// case we can simply extract it to a function
			// for example in `def` since we are going to
			// evaluate the value separately, we don't care
			// about recursion because we're going to handle
			// it wen we're evaluating the value. But in the
			// case of let it's a different story.
			if rawFirst.GetType() == ast.Symbol {
				sform = rawFirst.(*Symbol).GetName()
			}

			switch sform {
			case "def":
				ret, err = Def(rt, scope, list.Rest().(*List))
				break tco // return

			case "fn":
				ret, err = Fn(rt, scope, list.Rest().(*List))
				break tco // return

			// `if` evaluation rules:
			// * It has to get only 3 arguments: PRED THEN ELSE
			// * Evaluate only the PRED expression if the result
			//   is not `nil` or `false` evaluates THEN otherwise
			//   evaluate the ELSE expression and return the result.
			case "if":
				args := list.Rest().(*List)
				if args.Count() != 3 {
					return nil, MakeError(rt, "'if' needs exactly 3 aruments")
				}

				pred, err := EvalForms(rt, scope, args.First())
				result := pred.GetType()

				if err != nil {
					return nil, err
				}

				if result != ast.False && result != ast.Nil {
					// Truthy clause
					expressions = args.Rest().First()
				} else {

					// Falsy clause
					expressions = args.Rest().Rest().First()
				}

				continue tco // Loop over to execute the new expressions

			// `do` evaluation rules:
			// * Evaluate the body as a new block in the TCO loop
			//   and return the result of the last expression
			case "do":
				expressions = MakeBlock(list.Rest().(*List).ToSlice())
				continue tco // Loop over to execute the new expressions

			case "let":
				if list.Count() < 2 {
					return nil, MakeError(rt, "'let' needs at list 1 aruments")
				}

				letScope := MakeScope(scope.(*Scope))

				// Since we're using IColl for the bindings, we can use either lists
				// or vectors or even hashmaps for bindings
				var bindings IColl
				bindings = list.Rest().First().(IColl)

				body := list.Rest().Rest().(*List).ToSlice()

				if bindings.Count()%2 != 0 {
					return nil, MakeError(rt, "'let' bindings has to have even number of forms.")
				}

				for {
					// We're reducing over bindings here
					if bindings.Count() == 0 {
						break
					}

					name := bindings.First()
					expr := bindings.Rest().First()

					// TODO: We need to destruct the bindings here and remove this check
					//       for the symbol type
					if name.GetType() != ast.Symbol {
						err := MakeErrorFor(rt, name, "'let' doesn't support desbbtructuring yet, use a symbol.")
						return nil, err
					}

					// You might be wondering why we're using `EvalForms` here to evaluate
					// the exprs in bindings, what about TCO ?
					// Well, It's called TAIL call optimization for a reason. Exprs in the
					// bindings are not tail calls
					evaluatedExpr, e := EvalForms(rt, letScope, expr)

					if e != nil {
						return nil, e
					}

					letScope.Insert(name.String(), evaluatedExpr, false)
					bindings = bindings.Rest().Rest().(IColl)
				}

				expressions = MakeBlock(body)
				scope = letScope
				continue tco

			// list evaluation rules:
			// * The first element of the list has to be an expression which is callable
			// * An empty list evaluates to itself.
			default:
				// Evaluating all the elements of the list
				exprs, e := evalForm(rt, scope, list)
				if e != nil {
					err = e
					ret = nil
					break tco //return
				}

				f := exprs.(*List).First()

				switch f.GetType() {
				case ast.Fn:
					// If the first element of the evaluated list is a function
					// create a scope for it by creating the binding to the given
					// parameters in the new scope and set the parent of it to
					// the scope which the function defined in and then set the
					// `expressions` to the body of function and loop again
					fn := f.(*Function)
					if e != nil {
						err = e
						ret = nil
						break body //return

					}

					argList := exprs.(*List).Rest().(*List)

					scope, e = MakeFnScope(rt, fn.GetScope(), fn.GetParams(), argList)
					if e != nil {
						err = e
						ret = nil
						break body //return
					}

					expressions = fn.GetBody()
					continue tco
				default:
					err = MakeError(rt, "don't know how to execute anything beside function")
					ret = nil
					break tco
				}
			}
		}
	}

	return ret, err
}

// Eval the given `Block` of code with the given runtime `rt`.
// The Important part here is that any expression that we need
// to Eval has to be wrapped in a Block. Don't confused the
// concept of Block with blocks from other languages which
// specify by using `{}` or indent or what ever. Blocks in terms
// of Serene are just arrays of expressions and nothing more.
func Eval(rt *Runtime, forms *Block) (IExpr, IError) {
	if forms.Count() == 0 {
		// Nothing is literally Nothing
		return &Nothing, nil
	}

	v, err := EvalForms(rt, rt.CurrentNS().GetRootScope(), forms)

	if err != nil {
		return nil, err
	}

	return v, nil
}