path: root/boot/tree.c

/*
 * abstract syntax tree types for a catskill source.
 *
 * Copyright (c) 2025, Mel G. <mel@rnrd.eu>
 *
 * SPDX-License-Identifier: MPL-2.0
 */

#pragma once

#include "catboot.h"

enum Unary_Operation
{
    UNARY_NONE,

    UNARY_MINUS,
    UNARY_NOT,

    UNARY_BITWISE_NOT,
};

enum Unary_Operation
unary_operation_from_token(const struct Token* token)
{
    switch (token->kind) {
    case TOKEN_MINUS:
        return UNARY_MINUS;
    case TOKEN_BANG:
        return UNARY_NOT;
    case TOKEN_TILDE:
        return UNARY_BITWISE_NOT;

    default:
        return UNARY_NONE;
    }
}

const ascii*
unary_operation_to_string(enum Unary_Operation operation)
{
    switch (operation) {
    case UNARY_MINUS:
        return "-";
    case UNARY_NOT:
        return "!";
    case UNARY_BITWISE_NOT:
        return "~";

    default:
        failure("unexpected unary operation passed to `unary_operation_to_string`");
        return nil;
    }
}

enum Binary_Operation
{
    BINARY_NONE,

    BINARY_PLUS,
    BINARY_MINUS,
    BINARY_MULTIPLY,
    BINARY_DIVIDE,
    BINARY_MODULO,
    BINARY_POWER,

    BINARY_EQUAL,
    BINARY_NOT_EQUAL,
    BINARY_GREATER_THAN,
    BINARY_GREATER_THAN_EQUAL,
    BINARY_LESS_THAN,
    BINARY_LESS_THAN_EQUAL,
    BINARY_AND,
    BINARY_OR,

    BINARY_RANGE,

    BINARY_BITWISE_AND,
    BINARY_BITWISE_OR,
    BINARY_BITWISE_XOR,
    BINARY_BITWISE_LEFT_SHIFT,
    BINARY_BITWISE_RIGHT_SHIFT,

    BINARY_ASSIGN,
    BINARY_ASSIGN_PLUS,
    BINARY_ASSIGN_MINUS,
    BINARY_ASSIGN_MULTIPLY,
    BINARY_ASSIGN_DIVIDE,
    BINARY_ASSIGN_MODULO,
    BINARY_ASSIGN_AND,
    BINARY_ASSIGN_OR,
    BINARY_ASSIGN_BITWISE_AND,
    BINARY_ASSIGN_BITWISE_OR,
    BINARY_ASSIGN_BITWISE_XOR,
    BINARY_ASSIGN_BITWISE_LEFT_SHIFT,
    BINARY_ASSIGN_BITWISE_RIGHT_SHIFT,
};

enum Binary_Operation
binary_operation_from_token(const struct Token* token)
{
    switch (token->kind) {
    case TOKEN_PLUS:
        return BINARY_PLUS;
    case TOKEN_MINUS:
        return BINARY_MINUS;
    case TOKEN_STAR:
        return BINARY_MULTIPLY;
    case TOKEN_SLASH:
        return BINARY_DIVIDE;
    case TOKEN_PERCENT:
        return BINARY_MODULO;
    case TOKEN_STAR_STAR:
        return BINARY_POWER;

    case TOKEN_EQUAL:
        return BINARY_EQUAL;
    case TOKEN_NOT_EQUAL:
        return BINARY_NOT_EQUAL;
    case TOKEN_GREATER:
        return BINARY_GREATER_THAN;
    case TOKEN_GREATER_EQUAL:
        return BINARY_GREATER_THAN_EQUAL;
    case TOKEN_LESS:
        return BINARY_LESS_THAN;
    case TOKEN_LESS_EQUAL:
        return BINARY_LESS_THAN_EQUAL;
    case TOKEN_AND:
        return BINARY_AND;
    case TOKEN_OR:
        return BINARY_OR;

    case TOKEN_DOT_DOT:
        return BINARY_RANGE;

    case TOKEN_AMPERSAND:
        return BINARY_BITWISE_AND;
    case TOKEN_PIPE:
        return BINARY_BITWISE_OR;
    case TOKEN_CARET:
        return BINARY_BITWISE_XOR;
    case TOKEN_LEFT_SHIFT:
        return BINARY_BITWISE_LEFT_SHIFT;
    case TOKEN_RIGHT_SHIFT:
        return BINARY_BITWISE_RIGHT_SHIFT;

    case TOKEN_ASSIGN:
        return BINARY_ASSIGN;
    case TOKEN_ASSIGN_PLUS:
        return BINARY_ASSIGN_PLUS;
    case TOKEN_ASSIGN_MINUS:
        return BINARY_ASSIGN_MINUS;
    case TOKEN_ASSIGN_STAR:
        return BINARY_ASSIGN_MULTIPLY;
    case TOKEN_ASSIGN_SLASH:
        return BINARY_ASSIGN_DIVIDE;
    case TOKEN_ASSIGN_PERCENT:
        return BINARY_ASSIGN_MODULO;
    case TOKEN_ASSIGN_AND:
        return BINARY_ASSIGN_AND;
    case TOKEN_ASSIGN_OR:
        return BINARY_ASSIGN_OR;
    case TOKEN_ASSIGN_AMPERSAND:
        return BINARY_ASSIGN_BITWISE_AND;
    case TOKEN_ASSIGN_PIPE:
        return BINARY_ASSIGN_BITWISE_OR;
    case TOKEN_ASSIGN_CARET:
        return BINARY_ASSIGN_BITWISE_XOR;
    case TOKEN_ASSIGN_LEFT_SHIFT:
        return BINARY_ASSIGN_BITWISE_LEFT_SHIFT;
    case TOKEN_ASSIGN_RIGHT_SHIFT:
        return BINARY_ASSIGN_BITWISE_RIGHT_SHIFT;

    default:
        return BINARY_NONE;
    }
}

// return the precedence of the given binary operation.
// lower values are weaker, with 1 being the weakest.
// the values are taken mostly the same as other C-family languages.
uint
binary_operation_precedence(enum Binary_Operation operation)
{
    switch (operation) {
    // weakest
    case BINARY_ASSIGN:
    case BINARY_ASSIGN_PLUS:
    case BINARY_ASSIGN_MINUS:
    case BINARY_ASSIGN_MULTIPLY:
    case BINARY_ASSIGN_DIVIDE:
    case BINARY_ASSIGN_MODULO:
    case BINARY_ASSIGN_AND:
    case BINARY_ASSIGN_OR:
    case BINARY_ASSIGN_BITWISE_AND:
    case BINARY_ASSIGN_BITWISE_OR:
    case BINARY_ASSIGN_BITWISE_XOR:
    case BINARY_ASSIGN_BITWISE_LEFT_SHIFT:
    case BINARY_ASSIGN_BITWISE_RIGHT_SHIFT:
        return 1;
    case BINARY_RANGE:
        return 2;
    case BINARY_OR:
        return 3;
    case BINARY_AND:
        return 4;
    case BINARY_BITWISE_OR:
        return 5;
    case BINARY_BITWISE_XOR:
        return 6;
    case BINARY_BITWISE_AND:
        return 7;
    case BINARY_EQUAL:
    case BINARY_NOT_EQUAL:
        return 8;
    case BINARY_GREATER_THAN:
    case BINARY_GREATER_THAN_EQUAL:
    case BINARY_LESS_THAN:
    case BINARY_LESS_THAN_EQUAL:
        return 9;
    case BINARY_BITWISE_LEFT_SHIFT:
    case BINARY_BITWISE_RIGHT_SHIFT:
        return 10;
    case BINARY_PLUS:
    case BINARY_MINUS:
        return 11;
    case BINARY_MULTIPLY:
    case BINARY_DIVIDE:
    case BINARY_MODULO:
        return 12;
    case BINARY_POWER:
        return 13;
        // strongest

    default:
        failure("unexpected binary operation passed to `binary_operation_precedence`");
        return 0;
    }
}

enum Binary_Operation_Associativity
{
    BINARY_ASSOCIATIVITY_NONE,
    BINARY_ASSOCIATIVITY_LEFT,
    BINARY_ASSOCIATIVITY_RIGHT,
};

enum Binary_Operation_Associativity
binary_operation_associativity(enum Binary_Operation operation)
{
    switch (operation) {
    case BINARY_ASSIGN:
    case BINARY_ASSIGN_PLUS:
    case BINARY_ASSIGN_MINUS:
    case BINARY_ASSIGN_MULTIPLY:
    case BINARY_ASSIGN_DIVIDE:
    case BINARY_ASSIGN_MODULO:
    case BINARY_ASSIGN_AND:
    case BINARY_ASSIGN_OR:
    case BINARY_ASSIGN_BITWISE_AND:
    case BINARY_ASSIGN_BITWISE_OR:
    case BINARY_ASSIGN_BITWISE_XOR:
    case BINARY_ASSIGN_BITWISE_LEFT_SHIFT:
    case BINARY_POWER:
        return BINARY_ASSOCIATIVITY_RIGHT;
    case BINARY_RANGE:
        return BINARY_ASSOCIATIVITY_NONE;
    default:
        return BINARY_ASSOCIATIVITY_LEFT;
    }
}

const ascii*
binary_operation_to_string(enum Binary_Operation operation)
{
    switch (operation) {
    case BINARY_PLUS:
        return "+";
    case BINARY_MINUS:
        return "-";
    case BINARY_MULTIPLY:
        return "*";
    case BINARY_DIVIDE:
        return "/";
    case BINARY_MODULO:
        return "%";
    case BINARY_POWER:
        return "**";

    case BINARY_EQUAL:
        return "==";
    case BINARY_NOT_EQUAL:
        return "!=";
    case BINARY_GREATER_THAN:
        return ">";
    case BINARY_GREATER_THAN_EQUAL:
        return ">=";
    case BINARY_LESS_THAN:
        return "<";
    case BINARY_LESS_THAN_EQUAL:
        return "<=";
    case BINARY_AND:
        return "&&";
    case BINARY_OR:
        return "||";

    case BINARY_RANGE:
        return "..";

    case BINARY_BITWISE_AND:
        return "&";
    case BINARY_BITWISE_OR:
        return "|";
    case BINARY_BITWISE_XOR:
        return "^";
    case BINARY_BITWISE_LEFT_SHIFT:
        return "<<";
    case BINARY_BITWISE_RIGHT_SHIFT:
        return ">>";

    case BINARY_ASSIGN:
        return "=";
    case BINARY_ASSIGN_PLUS:
        return "+=";
    case BINARY_ASSIGN_MINUS:
        return "-=";
    case BINARY_ASSIGN_MULTIPLY:
        return "*=";
    case BINARY_ASSIGN_DIVIDE:
        return "/=";
    case BINARY_ASSIGN_MODULO:
        return "%=";
    case BINARY_ASSIGN_AND:
        return "&&=";
    case BINARY_ASSIGN_OR:
        return "||=";
    case BINARY_ASSIGN_BITWISE_AND:
        return "&=";
    case BINARY_ASSIGN_BITWISE_OR:
        return "|=";
    case BINARY_ASSIGN_BITWISE_XOR:
        return "^=";
    case BINARY_ASSIGN_BITWISE_LEFT_SHIFT:
        return "<<=";
    case BINARY_ASSIGN_BITWISE_RIGHT_SHIFT:
        return ">>=";

    default:
        failure("unexpected binary operation passed to `binary_operation_to_string`");
        return nil;
    }
}

enum Increment_Decrement_Operation
{
    INCREMENT_DECREMENT_NONE,
    INCREMENT_DECREMENT_INCREMENT,
    INCREMENT_DECREMENT_DECREMENT,
};

enum Increment_Decrement_Operation
increment_decrement_operation_from_token(const struct Token* token)
{
    switch (token->kind) {
    case TOKEN_PLUS_PLUS:
        return INCREMENT_DECREMENT_INCREMENT;
    case TOKEN_MINUS_MINUS:
        return INCREMENT_DECREMENT_DECREMENT;

    default:
        return INCREMENT_DECREMENT_NONE;
    }
}

const ascii*
increment_decrement_operation_to_string(enum Increment_Decrement_Operation operation)
{
    switch (operation) {
    case INCREMENT_DECREMENT_INCREMENT:
        return "++";
    case INCREMENT_DECREMENT_DECREMENT:
        return "--";

    default:
        failure("unexpected increment/decrement operation passed to "
                "`increment_decrement_operation_to_string`");
        return nil;
    }
}

// nodes are parts of the syntax tree that are reused often
// and in different places.

// a block of code, enclosed in curly braces.
// represents a sequence of statements that are executed in order.
struct Block_Node
{
    struct Statement* statements;
    struct Span span;
    struct Cursor location;
};

void block_node_print(const struct Block_Node* block);

// a function header, describing the parameters and return type of function.
// used both as a type and in full function definitions.
struct Function_Header_Node
{
    // linked list of parameters.
    // name, if given, is included in the type node.
    struct Type_Node* parameters_type_and_name;
    struct Type_Node* return_type;

    struct Span span;
};

void function_header_node_print(const struct Function_Header_Node* header);

// a declaration of a variable, constant, or other binding, without a mutability
// signifier, like `let` or `var`.
// the mutability is determined by some outside context, where
// a bare declaration in a for-loop, for example, is always mutable.
struct Bare_Declaration_Node
{
    struct String_Array names;
    struct Expression* initializer;
    struct Type_Node* type;

    struct Span span;
    struct Cursor location;
};

void bare_declaration_node_print(const struct Bare_Declaration_Node* declaration);

enum Type_Node_Type
{
    TYPE_NODE_NONE,

    TYPE_NODE_NAME,
    TYPE_NODE_ARRAY,     // an array of a type, `[int]`.
    TYPE_NODE_REFERENCE, // a reference to a type, `&int`.
    TYPE_NODE_MAYBE,     // a type that may be null, `int?`.
    TYPE_NODE_TUPLE,     // a tuple type, `(int string)`.
    TYPE_NODE_MAP,       // a map type, `[string = int]`.

    TYPE_NODE_FUNCTION,  // a function type, `fun (int) int`.
    TYPE_NODE_STRUCTURE, // a struct, invoked either with `type` or with `{}` when a type is inline.
    TYPE_NODE_VARIANT,   // a tagged union.
    TYPE_NODE_CLASS,     // a class of types, a.k.a. an interface.
};

struct Type_Node_Name
{
    struct String name;
};

struct Type_Node_Array
{
    struct Type_Node* element_type;
};

struct Type_Node_Reference
{
    struct Type_Node* referenced_type;
};

struct Type_Node_Maybe
{
    struct Type_Node* inner_type;
};

struct Type_Node_Tuple
{
    struct Type_Node* head; // the first type in the tuple, if any.
};

struct Type_Node_Map
{
    struct Type_Node* key_type;
    struct Type_Node* value_type;
};

struct Type_Node_Function
{
    struct Function_Header_Node header;
};

struct Type_Node_Structure
{
    // the fields of the structure, linked list of types and (required) names.
    struct Type_Node* fields;
};

struct Type_Node_Variant
{
    // the variants of the tagged union, linked list of (required) variant names and backing types.
    // if a variant has no backing type, it is TYPE_NODE_NONE.
    struct Type_Node* variants;
};

struct Type_Node_Class
{
    // linked list of the types of methods required to implement the class.
    // each node is required to have a name and be of TYPE_NODE_FUNCTION.
    struct Type_Node* methods;
};

union Type_Node_Value
{
    struct Type_Node_Name name;
    struct Type_Node_Array array;
    struct Type_Node_Reference reference;
    struct Type_Node_Maybe maybe;
    struct Type_Node_Tuple tuple;
    struct Type_Node_Map map;
    struct Type_Node_Function function;
    struct Type_Node_Structure structure;
    struct Type_Node_Variant variant;
    struct Type_Node_Class class;
};

// a type node represents a type in the syntax tree.
// currently, we only support types that are simple names.
// or null types.
// also includes the name of the field, member, parameter, etc., for which
// the type is defined.
struct Type_Node
{
    enum Type_Node_Type type;
    union Type_Node_Value value;
    // note: we could also just include the token here i think?
    struct Span span;
    struct Cursor location;

    // usually a type is preceded by the name of the binding
    // it is assigned to, e.g. `x int`, `string y`, etc.
    // this is the name of that binding, for ease of listing.
    struct String value_name;

    // if type is within a group of multiple types,
    // points to the next type within the group.
    struct Type_Node* next;
};

REGION(struct Type_Node, type_node)

// allocates a new type node in the global type node region.
struct Type_Node*
type_node_new(
    enum Type_Node_Type type, union Type_Node_Value value, struct Span span, struct Cursor location)
{
    check(region_type_node_cursor < REGION_SIZE, "out of type node memory");
    struct Type_Node* type_node = &region_type_node[region_type_node_cursor++];
    *type_node = (struct Type_Node){
        .type = type,
        .value = value,
        .span = span,
        .location = location,

        .value_name = string_empty(),
        .next = nil,
    };
    return type_node;
}

// allocates a new type node with no value, used for `none` types.
// this is used for types that are not specified, note that it is still
// fully allocated and can be used in the syntax tree.
struct Type_Node*
type_node_none(struct Span span, struct Cursor location)
{
    return type_node_new(TYPE_NODE_NONE, (union Type_Node_Value){ 0 }, span, location);
}

bool
type_node_is_none(const struct Type_Node* type_node)
{
    return type_node->type == TYPE_NODE_NONE;
}

void
type_node_print(const struct Type_Node* type_node)
{
    printf("(type ");
    switch (type_node->type) {
    case TYPE_NODE_NONE:
        printf("none");
        break;
    case TYPE_NODE_NAME:
        printf("name %s", type_node->value.name.name.data);
        break;
    case TYPE_NODE_ARRAY:
        printf("array of ");
        type_node_print(type_node->value.array.element_type);
        break;
    case TYPE_NODE_REFERENCE:
        printf("reference to ");
        type_node_print(type_node->value.reference.referenced_type);
        break;
    case TYPE_NODE_MAYBE:
        printf("maybe ");
        type_node_print(type_node->value.maybe.inner_type);
        break;
    case TYPE_NODE_TUPLE: {
        printf("tuple");
        FOR_EACH (struct Type_Node*, current, type_node->value.tuple.head) {
            printf(" ");
            type_node_print(current);
        }
        break;
    }
    case TYPE_NODE_MAP:
        printf("map ");
        type_node_print(type_node->value.map.key_type);
        printf(" = ");
        type_node_print(type_node->value.map.value_type);
        break;
    case TYPE_NODE_FUNCTION: {
        printf("function ");
        function_header_node_print(&type_node->value.function.header);
        break;
    }
    case TYPE_NODE_STRUCTURE: {
        printf("structure");
        FOR_EACH (struct Type_Node*, current, type_node->value.structure.fields) {
            printf(" (field %s) ", current->value_name.data);
            type_node_print(current);
        }
        break;
    }
    case TYPE_NODE_VARIANT: {
        printf("variant");
        FOR_EACH (struct Type_Node*, current, type_node->value.variant.variants) {
            if (type_node_is_none(current)) {
                printf(" (variant %s)", current->value_name.data);
            } else {
                printf(" (variant %s of ", current->value_name.data);
                type_node_print(current);
                printf(")");
            }
        }
        break;
    }
    case TYPE_NODE_CLASS:
        printf("class");
        FOR_EACH (struct Type_Node*, current, type_node->value.class.methods) {
            check(current->type == TYPE_NODE_FUNCTION,
                  "expected class method type node to be a function type");
            printf(" (method %s ", current->value_name.data);
            function_header_node_print(&current->value.function.header);
            printf(")");
        }
        break;
    }
    printf(")");
}

enum Expression_Kind
{
    EXPRESSION_NONE,

    EXPRESSION_INTEGER_LITERAL,
    EXPRESSION_FLOAT_LITERAL,
    EXPRESSION_STRING_LITERAL,
    EXPRESSION_BOOLEAN_LITERAL,
    EXPRESSION_NAME,

    EXPRESSION_UNARY_OPERATION,
    EXPRESSION_BINARY_OPERATION,

    EXPRESSION_GROUP,
    EXPRESSION_CALL_OR_CONSTRUCT,
    EXPRESSION_SUBSCRIPT,
    EXPRESSION_MEMBER,
    EXPRESSION_INCREMENT_DECREMENT,

    EXPRESSION_FUNCTION,
    EXPRESSION_TYPE,
};

struct Expression_Integer_Literal
{
    int64 value; // might not fit entire number given in source.
};

struct Expression_Float_Literal
{
    float64 value;
};

struct Expression_String_Literal
{
    struct String value;
};

struct Expression_Bool_Literal
{
    bool value;
};

struct Expression_Name
{
    struct String name;
};

struct Expression_Unary_Operator
{
    enum Unary_Operation operation;
    struct Expression* operand;
};

struct Expression_Binary_Operator
{
    enum Binary_Operation operation;
    struct Expression* left_operand;
    struct Expression* right_operand;
};

struct Expression_Group
{
    struct Expression* inner_expression;
};

struct Expression_Call_Or_Construct
{
    struct Expression* subject;
    // linked list of argument expressions.
    struct Expression* arguments;
    // names of the arguments, if given.
    // an unnamed argument is represented as an empty string.
    struct String_Array argument_names;
};

struct Expression_Subscript
{
    struct Expression* subject;
    struct Expression* index;
};

struct Expression_Member
{
    struct Expression* subject;
    struct String name;
};

struct Expression_Increment_Decrement
{
    // whether the increment/decrement is a prefix or postfix operation.
    bool prefix;
    struct Expression* subject;
    enum Increment_Decrement_Operation operation;
};

struct Expression_Function
{
    struct Function_Header_Node header;
    struct Block_Node body;
};

struct Expression_Type
{
    struct Type_Node* type;
};

union Expression_Value
{
    struct Expression_Integer_Literal integer_literal;
    struct Expression_Float_Literal float_literal;
    struct Expression_String_Literal string_literal;
    struct Expression_Bool_Literal bool_literal;
    struct Expression_Name name;
    struct Expression_Unary_Operator unary_operator;
    struct Expression_Binary_Operator binary_operator;
    struct Expression_Group group;
    struct Expression_Call_Or_Construct call_or_construct;
    struct Expression_Subscript subscript;
    struct Expression_Member member;
    struct Expression_Increment_Decrement increment_decrement;
    struct Expression_Function function;
    struct Expression_Type type;
};

struct Expression
{
    enum Expression_Kind kind;
    union Expression_Value value;

    struct Span span;
    struct Cursor location;

    // if expression is within a group of multiple expressions,
    // points to the next expression within it.
    struct Expression* next;
};

REGION(struct Expression, expression)

struct Expression*
expression_new(
    enum Expression_Kind kind, union Expression_Value value, struct Span span,
    struct Cursor location)
{
    check(region_expression_cursor < REGION_SIZE, "out of expression memory");
    struct Expression* expression = &region_expression[region_expression_cursor++];
    *expression = (struct Expression){
        .kind = kind,
        .value = value,
        .span = span,
        .location = location,
        .next = nil,
    };
    return expression;
}

void
function_header_node_print(const struct Function_Header_Node* header)
{
    FOR_EACH (struct Type_Node*, current, header->parameters_type_and_name) {
        if (current != header->parameters_type_and_name) printf(" ");

        if (!string_is_empty(current->value_name))
            printf("(param %s) ", current->value_name.data);
        else
            printf("(param) ");

        type_node_print(current);
    }

    if (header->return_type) {
        printf(" (returns ");
        type_node_print(header->return_type);
        printf(")");
    }
}

void
expression_print(const struct Expression* expression)
{
    printf("(expr ");
    switch (expression->kind) {
    case EXPRESSION_NONE:
        printf("none");
        break;
    case EXPRESSION_INTEGER_LITERAL:
        printf("%ld", expression->value.integer_literal.value);
        break;
    case EXPRESSION_FLOAT_LITERAL:
        printf("%lf", expression->value.float_literal.value);
        break;
    case EXPRESSION_STRING_LITERAL:
        printf("\"%s\"", expression->value.string_literal.value.data);
        break;
    case EXPRESSION_BOOLEAN_LITERAL:
        printf("%s", expression->value.bool_literal.value ? "true" : "false");
        break;
    case EXPRESSION_NAME:
        printf("(name %s)", expression->value.name.name.data);
        break;
    case EXPRESSION_UNARY_OPERATION:
        printf("(unary %s ", unary_operation_to_string(expression->value.unary_operator.operation));
        expression_print(expression->value.unary_operator.operand);
        printf(")");
        break;
    case EXPRESSION_BINARY_OPERATION:
        printf(
            "(binary %s ", binary_operation_to_string(expression->value.binary_operator.operation));
        expression_print(expression->value.binary_operator.left_operand);
        printf(" ");
        expression_print(expression->value.binary_operator.right_operand);
        printf(")");
        break;
    case EXPRESSION_GROUP:
        printf("(group ");
        expression_print(expression->value.group.inner_expression);
        printf(")");
        break;
    case EXPRESSION_CALL_OR_CONSTRUCT: {
        const struct Expression_Call_Or_Construct* coc = &expression->value.call_or_construct;
        printf("(call/construct ");
        expression_print(coc->subject);
        uint i = 0;
        FOR_EACH (struct Expression*, argument, coc->arguments) {
            struct String name = string_array_at(&coc->argument_names, i++);
            if (!string_is_empty(name)) {
                printf(" (named arg '%s' ", name.data);
            } else {
                printf(" (arg ");
            }
            expression_print(argument);
            printf(")");
        }
        printf(")");
        break;
    }
    case EXPRESSION_SUBSCRIPT:
        printf("(subscript ");
        expression_print(expression->value.subscript.subject);
        printf(" ");
        expression_print(expression->value.subscript.index);
        printf(")");
        break;
    case EXPRESSION_MEMBER:
        printf("(member of ");
        expression_print(expression->value.member.subject);
        printf(" named %s", expression->value.member.name.data);
        printf(")");
        break;
    case EXPRESSION_INCREMENT_DECREMENT: {
        const struct Expression_Increment_Decrement* inc_dec =
            &expression->value.increment_decrement;
        const ascii* prefix_or_postfix = inc_dec->prefix ? "prefix" : "postfix";
        printf("(increment/decrement %s %s ",
               increment_decrement_operation_to_string(inc_dec->operation), prefix_or_postfix);
        expression_print(inc_dec->subject);
        break;
    }
    case EXPRESSION_FUNCTION: {
        const struct Expression_Function* fun = &expression->value.function;
        printf("(function ");
        function_header_node_print(&fun->header);
        printf(" ");
        block_node_print(&fun->body);
        printf(")");
        break;
    }
    case EXPRESSION_TYPE:
        type_node_print(expression->value.type.type);
        break;
    default:
        failure("unexpected expression kind passed to `expression_print`");
        break;
    }
    printf(")");
}

enum Statement_Kind
{
    STATEMENT_NONE,
    STATEMENT_EXPRESSION,
    STATEMENT_DECLARATION,
    // NOTE: a block could be an expression in the future.
    STATEMENT_BLOCK,
    STATEMENT_CONDITIONAL,
    STATEMENT_LOOP,
    STATEMENT_RETURN,
    STATEMENT_BREAK,
    STATEMENT_CONTINUE,
    STATEMENT_DEFER,
};

struct Statement_Value_Expression
{
    struct Expression* inner;
};

enum Statement_Declaration_Kind
{
    STATEMENT_DECLARATION_NONE,
    STATEMENT_DECLARATION_VARIABLE,
    STATEMENT_DECLARATION_CONSTANT,
};

enum Statement_Declaration_Kind
statement_declaration_kind_from_token(const struct Token* token)
{
    switch (token->kind) {
    case TOKEN_WORD_VAR:
        return STATEMENT_DECLARATION_VARIABLE;
    case TOKEN_WORD_LET:
        return STATEMENT_DECLARATION_CONSTANT;

    default:
        return STATEMENT_DECLARATION_NONE;
    }
}

struct Statement_Value_Declaration
{
    enum Statement_Declaration_Kind kind;
    struct Bare_Declaration_Node inner;
};

struct Statement_Value_Block
{
    struct Block_Node inner; // the block of statements.
};

#define STATEMENT_VALUE_CONDITIONAL_MAX 8

struct Statement_Value_Conditional
{
    struct Statement_Conditional_Branch
    {
        // if nil, the condition is always true.
        struct Expression* when;
        struct Block_Node then;
    } conditions[STATEMENT_VALUE_CONDITIONAL_MAX];
    uint condition_count;
};

enum Statement_Loop_Style
{
    STATEMENT_LOOP_STYLE_NONE,
    STATEMENT_LOOP_STYLE_C,        // for i int = 0; i < 10; ++i {}
    STATEMENT_LOOP_STYLE_FOR_EACH, // for x Obj = list {}
    STATEMENT_LOOP_STYLE_WHILE,    // while true {}
    STATEMENT_LOOP_STYLE_ENDLESS,  // while {}
};

// stands for both `for` and `while` loops.
struct Statement_Value_Loop
{
    enum Statement_Loop_Style style;
    struct Bare_Declaration_Node declaration;
    struct Expression* condition;
    struct Expression* iteration;

    struct Block_Node body;
};

struct Statement_Value_Return
{
    // nil if there is no return value.
    struct Expression* value;
};

struct Statement_Value_Defer
{
    // either a simple expression, or, if expression is nil,
    // a block of code to execute.
    struct Expression* expression;
    struct Block_Node block;
};

union Statement_Value
{
    struct Statement_Value_Expression expression;
    struct Statement_Value_Declaration declaration;
    struct Statement_Value_Block block;
    struct Statement_Value_Conditional conditional;
    struct Statement_Value_Loop loop;
    struct Statement_Value_Return return_value;
    struct Statement_Value_Defer defer;
};

struct Statement
{
    enum Statement_Kind kind;
    union Statement_Value value;

    struct Span span;
    struct Cursor location;

    // if statement is within a group of multiple statements,
    // points to the next statement within it.
    struct Statement* next;
};

REGION(struct Statement, statement)

void statement_print(const struct Statement* statement);

void
block_node_print(const struct Block_Node* block)
{
    printf("(block \n");
    FOR_EACH (struct Statement*, statement, block->statements) {
        printf("\t");
        statement_print(statement);
        printf("\n");
    }
    printf(")");
}

void
bare_declaration_node_print(const struct Bare_Declaration_Node* declaration)
{
    printf("(declaration ");
    STRING_ARRAY_FOR_EACH (i, name, declaration->names) { printf("%s ", name.data); }
    if (!type_node_is_none(declaration->type)) {
        type_node_print(declaration->type);
        printf(" ");
    }

    if (declaration->initializer) {
        printf("(initializer ");
        expression_print(declaration->initializer);
        printf(")");
    }
    printf(")");
}

struct Statement*
statement_new(
    enum Statement_Kind kind, union Statement_Value value, struct Span span, struct Cursor location)
{
    check(region_statement_cursor < REGION_SIZE, "out of statement memory");
    struct Statement* statement = &region_statement[region_statement_cursor++];
    *statement = (struct Statement){
        .kind = kind,
        .value = value,
        .span = span,
        .location = location,
        .next = nil,
    };
    return statement;
}

void
statement_print(const struct Statement* statement)
{
    printf("(stmt ");
    switch (statement->kind) {
    case STATEMENT_NONE:
        printf("none");
        break;
    case STATEMENT_EXPRESSION: {
        const struct Expression* expression = statement->value.expression.inner;
        expression_print(expression);
        break;
    }
    case STATEMENT_DECLARATION: {
        if (statement->value.declaration.kind == STATEMENT_DECLARATION_VARIABLE)
            printf("(variable ");
        else if (statement->value.declaration.kind == STATEMENT_DECLARATION_CONSTANT)
            printf("(constant ");

        bare_declaration_node_print(&statement->value.declaration.inner);
        printf(")");
        break;
    }
    case STATEMENT_BLOCK:
        block_node_print(&statement->value.block.inner);
        break;
    case STATEMENT_CONDITIONAL: {
        printf("(conditional");
        for (uint i = 0; i < statement->value.conditional.condition_count; ++i) {
            const struct Statement_Conditional_Branch* branch =
                &statement->value.conditional.conditions[i];

            printf(" ");
            if (branch->when) {
                printf("(when ");
                expression_print(branch->when);
                printf(") ");
            } else {
                printf("(always) ");
            }

            block_node_print(&branch->then);
        }
        printf(")");
        break;
    }
    case STATEMENT_LOOP: {
        printf("(loop ");

        switch (statement->value.loop.style) {
        case STATEMENT_LOOP_STYLE_C:
            printf("c-style ");
            bare_declaration_node_print(&statement->value.loop.declaration);
            printf(" (condition ");
            expression_print(statement->value.loop.condition);
            printf(") (iteration ");
            expression_print(statement->value.loop.iteration);
            printf(") ");
            break;
        case STATEMENT_LOOP_STYLE_FOR_EACH:
            printf("for-each ");
            bare_declaration_node_print(&statement->value.loop.declaration);
            printf(" ");
            break;
        case STATEMENT_LOOP_STYLE_WHILE:
            printf("while (condition ");
            expression_print(statement->value.loop.condition);
            printf(") ");
            break;
        case STATEMENT_LOOP_STYLE_ENDLESS:
            printf("endless ");
            break;
        default:
            failure("unexpected loop style in `statement_print`");
            break;
        }

        block_node_print(&statement->value.loop.body);
        printf(")");
        break;
    }
    case STATEMENT_RETURN: {
        printf("(return");
        if (statement->value.return_value.value) {
            printf(" ");
            expression_print(statement->value.return_value.value);
        }
        printf(")");
        break;
    }
    case STATEMENT_BREAK:
        printf("(break)");
        break;
    case STATEMENT_CONTINUE:
        printf("(continue)");
        break;
    case STATEMENT_DEFER: {
        printf("(defer ");
        if (statement->value.defer.expression) {
            expression_print(statement->value.defer.expression);
        } else {
            block_node_print(&statement->value.defer.block);
        }
        printf(")");
        break;
    }
    default:
        failure("unexpected statement kind passed to `statement_print`");
        break;
    }
    printf(")");
}

// the top-level tree of a single catskill source file.
struct Tree
{
    struct Statement* top_level_statements;
};

void
tree_print(const struct Tree* tree)
{
    FOR_EACH (struct Statement*, statement, tree->top_level_statements) {
        statement_print(statement);
        printf("\n");
    }
}