from tokens import *
from std import STD, evaluate_expression


def peek(inp):
    if len(inp) == 0:
        return None
    return inp[0]

def evaluate(items, pop):
    nxt = peek(items)
    if nxt is None:
        return pop
    elif isinstance(nxt, NebLiteral):
        return evaluate(items[1:], nxt)
    elif isinstance(nxt, NebSymbol):
        if not nxt.name in STD:
            raise Exception(f"no such symbol: '{nxt.name}'")
        this_func = STD[nxt.name]
        return evaluate(items[1:], this_func[0].impl) # TODO show all
    elif isinstance(nxt, NebExpression):
        return evaluate(items[1:], evaluate_expression(nxt))
    else:
        raise Exception("expected a literal or an expression")