schala/schala-lang/language/src/eval.rs

use std::cell::RefCell;
use std::rc::Rc;
use std::fmt::Write;
use std::io;

use itertools::Itertools;

use crate::util::ScopeStack;
use crate::reduced_ast::{BoundVars, ReducedAST, Stmt, Expr, Lit, Func, Alternative, Subpattern};
use crate::symbol_table::{SymbolSpec, Symbol, SymbolTable};
use crate::builtin::Builtin;

mod test;

pub struct State<'a> {
  values: ScopeStack<'a, Rc<String>, ValueEntry>,
  symbol_table_handle: Rc<RefCell<SymbolTable>>,
}

impl<'a> State<'a> {
  pub fn new(symbol_table_handle: Rc<RefCell<SymbolTable>>) -> State<'a> {
    let values = ScopeStack::new(Some(format!("global")));
    State { values, symbol_table_handle }
  }

  pub fn debug_print(&self) -> String {
    format!("Values: {:?}", self.values)
  }

  fn new_frame(&'a self, items: &'a Vec<Node>, bound_vars: &BoundVars) -> State<'a> {
    let mut inner_state = State {
      values: self.values.new_scope(None),
      symbol_table_handle: self.symbol_table_handle.clone(),
    };
    for (bound_var, val) in bound_vars.iter().zip(items.iter()) {
      if let Some(bv) = bound_var.as_ref() {
        inner_state.values.insert(bv.clone(), ValueEntry::Binding { constant: true, val: val.clone() });
      }
    }
    inner_state
  }
}

#[derive(Debug, Clone)]
enum Node {
  Expr(Expr),
  PrimObject {
    name: Rc<String>,
    tag: usize,
    items: Vec<Node>,
  },
  PrimTuple {
    items: Vec<Node>
  }
}

fn paren_wrapped_vec(terms: impl Iterator<Item=String>) -> String {
  let mut buf = String::new();
  write!(buf, "(").unwrap();
  for term in terms.map(|e| Some(e)).intersperse(None) {
    match term {
      Some(e) => write!(buf, "{}", e).unwrap(),
      None => write!(buf, ", ").unwrap(),
    };
  }
  write!(buf, ")").unwrap();
  buf
}


impl Node {
  fn to_repl(&self, symbol_table: &SymbolTable) -> String {
    match self {
      Node::Expr(e) => e.to_repl(symbol_table),
      Node::PrimObject { name, items, .. } if items.len() == 0 => format!("{}", name),
      Node::PrimObject { name, items, .. } => format!("{}{}", name, paren_wrapped_vec(items.iter().map(|x| x.to_repl(symbol_table)))),
      Node::PrimTuple { items } => format!("{}", paren_wrapped_vec(items.iter().map(|x| x.to_repl(symbol_table)))),
    }
  }
  fn is_true(&self) -> bool {
    match self {
      Node::Expr(Expr::Lit(crate::reduced_ast::Lit::Bool(true))) => true,
      _ => false,
    }
  }
}

#[derive(Debug)]
enum ValueEntry {
  Binding {
    constant: bool,
    val: /*FullyEvaluatedExpr*/ Node, //TODO make this use a subtype to represent fully evaluatedness
  }
}

type EvalResult<T> = Result<T, String>;

impl Expr {
  fn to_node(self) -> Node {
    Node::Expr(self)
  }
  fn to_repl(&self, symbol_table: &SymbolTable) -> String {
    use self::Lit::*;
    use self::Func::*;

    let _ = symbol_table;

    match self {
      Expr::Lit(ref l) => match l {
        Nat(n) => format!("{}", n),
        Int(i) => format!("{}", i),
        Float(f) => format!("{}", f),
        Bool(b) => format!("{}", b),
        StringLit(s) => format!("\"{}\"", s),
      },
      Expr::Func(f) => match f {
        BuiltIn(builtin) => format!("<built-in function '{:?}'>", builtin),
        UserDefined { name: None, .. } => format!("<function>"),
        UserDefined { name: Some(name), .. } => format!("<function '{}'>", name),
      },
      Expr::Constructor { type_name, arity, .. } => {
        format!("<constructor for `{}` arity {}>", type_name, arity)
      },
      Expr::Tuple(exprs) => paren_wrapped_vec(exprs.iter().map(|x| x.to_repl(symbol_table))),
      _ => format!("{:?}", self),
    }
  }

  fn replace_conditional_target_sigil(self, replacement: &Expr) -> Expr {
    use self::Expr::*;

    match self {
      ConditionalTargetSigilValue => replacement.clone(),
      Unit | Lit(_) | Func(_) | Val(_) | Constructor { .. } |
       CaseMatch { .. } | UnimplementedSigilValue | ReductionError(_) => self,
      Tuple(exprs) => Tuple(exprs.into_iter().map(|e| e.replace_conditional_target_sigil(replacement)).collect()),
      Call { f, args } => {
        let new_args = args.into_iter().map(|e| e.replace_conditional_target_sigil(replacement)).collect();
        Call { f, args: new_args }
      },
      Conditional { .. } => panic!("Dunno if I need this, but if so implement"),
      Assign { .. } => panic!("I'm pretty sure I don't need this"),
    }
  }
}

impl<'a> State<'a> {
  pub fn evaluate(&mut self, ast: ReducedAST, repl: bool) -> Vec<Result<String, String>> {
    let mut acc = vec![];

    // handle prebindings
    for statement in ast.0.iter() {
      self.prebinding(statement);
    }

    for statement in ast.0 {
      match self.statement(statement) {
        Ok(Some(ref output)) if repl => {
          let ref symbol_table = self.symbol_table_handle.borrow();
          acc.push(Ok(output.to_repl(symbol_table)))
        },
        Ok(_) => (),
        Err(error) => {
          acc.push(Err(format!("Runtime error: {}", error)));
          return acc;
        },
      }
    }
    acc
  }

  fn prebinding(&mut self, stmt: &Stmt) {
    match stmt {
      Stmt::PreBinding { name, func } => {
        let v_entry = ValueEntry::Binding { constant: true, val: Node::Expr(Expr::Func(func.clone())) };
        self.values.insert(name.clone(), v_entry);
      },
      Stmt::Expr(_expr) => {
        //TODO have this support things like nested function defs

      },
      _ => ()
    }
  }

  fn statement(&mut self, stmt: Stmt) -> EvalResult<Option<Node>> {
    match stmt {
      Stmt::Binding { name, constant, expr } => {
        let val = self.expression(Node::Expr(expr))?;
        self.values.insert(name.clone(), ValueEntry::Binding { constant, val });
        Ok(None)
      },
      Stmt::Expr(expr) => Ok(Some(self.expression(expr.to_node())?)),
      Stmt::PreBinding {..} | Stmt::Noop => Ok(None),
    }
  }

  fn block(&mut self, stmts: Vec<Stmt>) -> EvalResult<Node> {
    let mut ret = None;
    for stmt in stmts {
      ret = self.statement(stmt)?;
    }
    Ok(ret.unwrap_or(Node::Expr(Expr::Unit)))
  }

  fn expression(&mut self, node: Node) -> EvalResult<Node> {
    use self::Expr::*;
    match node {
      t @ Node::PrimTuple { .. } => Ok(t),
      obj @ Node::PrimObject { .. } =>  Ok(obj),
      Node::Expr(expr) => match expr {
        literal @ Lit(_) => Ok(Node::Expr(literal)),
        Call { box f, args } => self.call_expression(f, args),
        Val(v) => self.value(v),
        Constructor { arity, ref name, tag, .. } if arity == 0 => Ok(Node::PrimObject { name: name.clone(), tag, items: vec![] }),
        constructor @ Constructor { .. } => Ok(Node::Expr(constructor)),
        func @ Func(_) => Ok(Node::Expr(func)),
        Tuple(exprs) => {
          let nodes = exprs.into_iter().map(|expr| self.expression(Node::Expr(expr))).collect::<Result<Vec<Node>,_>>()?;
          Ok(Node::PrimTuple { items: nodes })
        },
        Conditional { box cond, then_clause, else_clause } => self.conditional(cond, then_clause, else_clause),
        Assign { box val, box expr } => self.assign_expression(val, expr),
        Unit => Ok(Node::Expr(Unit)),
        CaseMatch { box cond, alternatives } => self.case_match_expression(cond, alternatives),
        ConditionalTargetSigilValue => Ok(Node::Expr(ConditionalTargetSigilValue)),
        UnimplementedSigilValue => Err(format!("Sigil value eval not implemented")),
        ReductionError(err) => Err(format!("Reduction error: {}", err)),
      }
    }
  }

  fn call_expression(&mut self, f: Expr, args: Vec<Expr>) -> EvalResult<Node> {
    use self::Expr::*;
    match self.expression(Node::Expr(f))? {
      Node::Expr(Constructor { type_name, name, tag, arity }) => self.apply_data_constructor(type_name, name, tag, arity, args),
      Node::Expr(Func(f)) => self.apply_function(f, args),
      other => return Err(format!("Tried to call {:?} which is not a function or data constructor", other)),
    }
  }

  fn apply_data_constructor(&mut self, _type_name: Rc<String>, name: Rc<String>, tag: usize, arity: usize, args: Vec<Expr>) -> EvalResult<Node> {
    if arity != args.len() {
      return Err(format!("Data constructor {} requires {} arg(s)", name, arity));
    }

    let evaled_args = args.into_iter().map(|expr| self.expression(Node::Expr(expr))).collect::<Result<Vec<Node>,_>>()?;
    //let evaled_args = vec![];
    Ok(Node::PrimObject {
      name: name.clone(),
      items: evaled_args,
      tag 
    })
  }

  fn apply_function(&mut self, f: Func, args: Vec<Expr>) -> EvalResult<Node> {
    match f {
      Func::BuiltIn(builtin) => Ok(Node::Expr(self.apply_builtin(builtin, args)?)),
      Func::UserDefined { params, body, name } => {

        if params.len() != args.len() {
          return Err(format!("calling a {}-argument function with {} args", params.len(), args.len()))
        }
        let mut func_state = State {
          values: self.values.new_scope(name.map(|n| format!("{}", n))),
          symbol_table_handle: self.symbol_table_handle.clone(),
        };
        for (param, val) in params.into_iter().zip(args.into_iter()) {
          let val = func_state.expression(Node::Expr(val))?;
          func_state.values.insert(param, ValueEntry::Binding { constant: true, val });
        }
        // TODO figure out function return semantics
        func_state.block(body)
      }
    }
  }

  //TODO to make builtins work, need to change this concept of Node
  fn apply_builtin(&mut self, builtin: Builtin, args: Vec<Expr>) -> EvalResult<Expr> {
    use self::Expr::*;
    use self::Lit::*;
    use Builtin::*;

    let evaled_args: Result<Vec<Expr>, String> = args.into_iter().map(|arg| {
      match self.expression(Node::Expr(arg)) {
        Ok(Node::Expr(e)) => Ok(e),
        Ok(Node::PrimTuple { .. }) => Err(format!("Trying to apply a builtin to a tuple")),
        Ok(Node::PrimObject { .. }) => Err(format!("Trying to apply a builtin to a primitive object")),
        Err(e) => Err(e)
      }
    }).collect();
    let evaled_args = evaled_args?;

    Ok(match (builtin, evaled_args.as_slice()) {
      /* binops */
      (Add, &[Lit(Nat(l)), Lit(Nat(r))]) => Lit(Nat(l + r)),
      (Concatenate, &[Lit(StringLit(ref s1)), Lit(StringLit(ref s2))]) => Lit(StringLit(Rc::new(format!("{}{}", s1, s2)))),
      (Subtract, &[Lit(Nat(l)), Lit(Nat(r))]) => Lit(Nat(l - r)),
      (Multiply, &[Lit(Nat(l)), Lit(Nat(r))]) => Lit(Nat(l * r)),
      (Divide, &[Lit(Nat(l)), Lit(Nat(r))]) => Lit(Float((l as f64)/ (r as f64))),
      (Quotient, &[Lit(Nat(l)), Lit(Nat(r))]) => if r == 0 {
        return Err(format!("divide by zero"));
      } else {
        Lit(Nat(l / r))
      },
      (Modulo, &[Lit(Nat(l)), Lit(Nat(r))]) => Lit(Nat(l % r)),
      (Exponentiation, &[Lit(Nat(l)), Lit(Nat(r))]) => Lit(Nat(l ^ r)),
      (BitwiseAnd, &[Lit(Nat(l)), Lit(Nat(r))]) => Lit(Nat(l & r)),
      (BitwiseOr, &[Lit(Nat(l)), Lit(Nat(r))]) => Lit(Nat(l | r)),

      /* comparisons */
      (Equality, &[Lit(Nat(l)), Lit(Nat(r))]) => Lit(Bool(l == r)),
      (Equality, &[Lit(Int(l)), Lit(Int(r))]) => Lit(Bool(l == r)),
      (Equality, &[Lit(Float(l)), Lit(Float(r))]) => Lit(Bool(l == r)),
      (Equality, &[Lit(Bool(l)), Lit(Bool(r))]) => Lit(Bool(l == r)),
      (Equality, &[Lit(StringLit(ref l)), Lit(StringLit(ref r))]) => Lit(Bool(l == r)),

      (LessThan, &[Lit(Nat(l)), Lit(Nat(r))]) => Lit(Bool(l < r)),
      (LessThan, &[Lit(Int(l)), Lit(Int(r))]) => Lit(Bool(l < r)),
      (LessThan, &[Lit(Float(l)), Lit(Float(r))]) => Lit(Bool(l < r)),

      (LessThanOrEqual, &[Lit(Nat(l)), Lit(Nat(r))]) => Lit(Bool(l <= r)),
      (LessThanOrEqual, &[Lit(Int(l)), Lit(Int(r))]) => Lit(Bool(l <= r)),
      (LessThanOrEqual, &[Lit(Float(l)), Lit(Float(r))]) => Lit(Bool(l <= r)),

      (GreaterThan, &[Lit(Nat(l)), Lit(Nat(r))]) => Lit(Bool(l > r)),
      (GreaterThan, &[Lit(Int(l)), Lit(Int(r))]) => Lit(Bool(l > r)),
      (GreaterThan, &[Lit(Float(l)), Lit(Float(r))]) => Lit(Bool(l > r)),

      (GreaterThanOrEqual, &[Lit(Nat(l)), Lit(Nat(r))]) => Lit(Bool(l >= r)),
      (GreaterThanOrEqual, &[Lit(Int(l)), Lit(Int(r))]) => Lit(Bool(l >= r)),
      (GreaterThanOrEqual, &[Lit(Float(l)), Lit(Float(r))]) => Lit(Bool(l >= r)),

      /* prefix ops */
      (BooleanNot, &[Lit(Bool(true))]) => Lit(Bool(false)),
      (BooleanNot, &[Lit(Bool(false))]) => Lit(Bool(true)),
      (Negate, &[Lit(Nat(n))]) => Lit(Int(-1*(n as i64))),
      (Negate, &[Lit(Int(n))]) => Lit(Int(-1*(n as i64))),
      (Increment, &[Lit(Int(n))]) => Lit(Int(n)),
      (Increment, &[Lit(Nat(n))]) => Lit(Nat(n)),


      /* builtin functions */
      (IOPrint, &[ref anything]) => {
        let ref symbol_table = self.symbol_table_handle.borrow();
        print!("{}", anything.to_repl(symbol_table));
        Expr::Unit
      },
      (IOPrintLn, &[ref anything]) => {
        let ref symbol_table = self.symbol_table_handle.borrow();
        println!("{}", anything.to_repl(symbol_table));
        Expr::Unit
      },
      (IOGetLine, &[]) => {
        let mut buf = String::new();
        io::stdin().read_line(&mut buf).expect("Error readling line in 'getline'");
        Lit(StringLit(Rc::new(buf.trim().to_string())))
      },
      (x, args) => return Err(format!("bad or unimplemented builtin {:?} | {:?}", x, args)),
    })
  }

  fn conditional(&mut self, cond: Expr, then_clause: Vec<Stmt>, else_clause: Vec<Stmt>) -> EvalResult<Node> {
    let cond = self.expression(Node::Expr(cond))?;
    Ok(match cond {
      Node::Expr(Expr::Lit(Lit::Bool(true))) =>  self.block(then_clause)?,
      Node::Expr(Expr::Lit(Lit::Bool(false))) => self.block(else_clause)?,
      _ => return Err(format!("Conditional with non-boolean condition"))
    })
  }

  fn assign_expression(&mut self, val: Expr, expr: Expr) -> EvalResult<Node> {
    let name = match val  {
      Expr::Val(name) => name,
      _ => return Err(format!("Trying to assign to a non-value")),
    };

    let constant = match self.values.lookup(&name) {
      None => return Err(format!("Constant {} is undefined", name)),
      Some(ValueEntry::Binding { constant, .. }) => constant.clone(),
    };
    if constant {
      return Err(format!("trying to update {}, a non-mutable binding", name));
    }
    let val = self.expression(Node::Expr(expr))?;
    self.values.insert(name.clone(), ValueEntry::Binding { constant: false, val });
    Ok(Node::Expr(Expr::Unit))
  }

  fn guard_passes(&mut self, guard: &Option<Expr>, cond: &Node) -> EvalResult<bool> {
    if let Some(ref guard_expr) = guard {
      let guard_expr = match cond {
        Node::Expr(ref e) => guard_expr.clone().replace_conditional_target_sigil(e),
        _ => guard_expr.clone()
      };
      Ok(self.expression(guard_expr.to_node())?.is_true())
    } else {
      Ok(true)
    }
  }

  fn case_match_expression(&mut self, cond: Expr, alternatives: Vec<Alternative>) -> EvalResult<Node> {

    //TODO need to handle recursive subpatterns
    let all_subpatterns_pass = |state: &mut State, subpatterns: &Vec<Option<Subpattern>>, items: &Vec<Node>| -> EvalResult<bool> {

      if subpatterns.len() == 0 {
        return Ok(true)
      }

      if items.len() != subpatterns.len() {
        return Err(format!("Subpattern length isn't correct items {} subpatterns {}", items.len(), subpatterns.len()));
      }

      for (maybe_subp, cond) in subpatterns.iter().zip(items.iter()) {
        if let Some(subp) = maybe_subp {
          if !state.guard_passes(&subp.guard, &cond)? {
            return Ok(false)
          }
        }
      }
      Ok(true)
    };

    let cond = self.expression(Node::Expr(cond))?;
    for alt in alternatives {
      // no matter what type of condition we have, ignore alternative if the guard evaluates false
      if !self.guard_passes(&alt.matchable.guard, &cond)? {
        continue;
      }

      match cond {
        Node::PrimObject { ref tag, ref items, .. } => {
          if alt.matchable.tag.map(|t| t == *tag).unwrap_or(true) {
            let mut inner_state = self.new_frame(items, &alt.matchable.bound_vars);
            if all_subpatterns_pass(&mut inner_state, &alt.matchable.subpatterns, items)? {
              return inner_state.block(alt.item);
            } else {
              continue;
            }
          }
        },
        Node::PrimTuple { ref items } =>  {
          let mut inner_state = self.new_frame(items, &alt.matchable.bound_vars);
          if all_subpatterns_pass(&mut inner_state, &alt.matchable.subpatterns, items)? {
            return inner_state.block(alt.item);
          } else {
            continue;
          }
        },
        Node::Expr(ref _e) => {
          if let None = alt.matchable.tag {
            return self.block(alt.item)
          }
        }
      }
    }
    Err(format!("{:?} failed pattern match", cond))
  }

  //TODO if I don't need to lookup by name here...
  fn value(&mut self, name: Rc<String>) -> EvalResult<Node> {
    use self::ValueEntry::*;
    use self::Func::*;
    //TODO add a layer of indirection here to talk to the symbol table first, and only then look up
    //in the values table

    let symbol_table = self.symbol_table_handle.borrow();
    let value = symbol_table.lookup_by_name(&name);
    Ok(match value {
      Some(Symbol { name, spec, .. }) => match spec {
        //TODO I'll need this type_name later to do a table lookup
        SymbolSpec::DataConstructor { type_name: _type_name, type_args, .. } => {
          if type_args.len() == 0 {
            Node::PrimObject { name: name.clone(), tag: 0, items: vec![] }
          } else {
            return Err(format!("This data constructor thing not done"))
          }
        },
        SymbolSpec::Func(_) => match self.values.lookup(&name) {
          Some(Binding { val: Node::Expr(Expr::Func(UserDefined { name, params, body })), .. }) => {
            Node::Expr(Expr::Func(UserDefined { name: name.clone(), params: params.clone(), body: body.clone() }))
          },
          _ => unreachable!(),
        },
        SymbolSpec::RecordConstructor { .. } => return Err(format!("This shouldn't be a record!")),
        SymbolSpec::Binding => match self.values.lookup(&name) {
          Some(Binding { val, .. }) => val.clone(),
          None => return Err(format!("Symbol {} exists in symbol table but not in evaluator table", name))
        }
      },
      //TODO ideally this should be returning a runtime error if this is ever None, but it's not
      //handling all bindings correctly yet
      //None => return Err(format!("Couldn't find value {}", name)),
      None => match self.values.lookup(&name) {
        Some(Binding { val, .. }) => val.clone(),
        None => return Err(format!("Couldn't find value {}", name)),
      }
    })
  }
}