diff --git a/compiler/ast/ast.ts b/compiler/ast/ast.ts
new file mode 100644
index 0000000..4040f6e
--- /dev/null
+++ b/compiler/ast/ast.ts
@@ -0,0 +1,23 @@
+export type File = {
+    stmts: Stmt[];
+};
+
+export type Stmt = {
+    id: number;
+    kind: StmtKind;
+};
+
+export type StmtKind =
+    | { tag: "error" }
+    | { tag: "mod_block" } & ModBlockStmt
+    | { tag: "mod_file" } & ModFileStmt;
+
+export type ModBlockStmt = {
+    ident: string;
+    stmts: Stmt[];
+};
+
+export type ModFileStmt = {
+    ident: string;
+    filePath: string;
+};
diff --git a/compiler/ast/mod.ts b/compiler/ast/mod.ts
new file mode 100644
index 0000000..7627d46
--- /dev/null
+++ b/compiler/ast/mod.ts
@@ -0,0 +1,2 @@
+export * from "./ast.ts";
+export * from "./visitor.ts";
diff --git a/compiler/ast/visitor.ts b/compiler/ast/visitor.ts
new file mode 100644
index 0000000..7c4ed88
--- /dev/null
+++ b/compiler/ast/visitor.ts
@@ -0,0 +1,62 @@
+import { exhausted } from "../util.ts";
+import { File, ModBlockStmt, ModFileStmt, Stmt } from "./ast.ts";
+
+export type VisitRes = "stop" | void;
+
+type R = VisitRes;
+type PM = unknown[];
+
+export interface Visitor<
+    P extends PM = [],
+> {
+    visitFile?(file: File, ...p: P): R;
+    visitStmt?(stmt: Stmt, ...p: P): R;
+    visitErrorStmt?(stmt: Stmt, ...p: P): R;
+    visitModBlockStmt?(stmt: Stmt, kind: ModBlockStmt, ...p: P): R;
+    visitModFileStmt?(stmt: Stmt, kind: ModFileStmt, ...p: P): R;
+}
+
+export function visitFile<
+    P extends PM = [],
+>(
+    v: Visitor<P>,
+    file: File,
+    ...p: P
+) {
+    if (v.visitFile?.(file, ...p) === "stop") return;
+    visitStmts(v, file.stmts, ...p);
+}
+
+export function visitStmts<
+    P extends PM = [],
+>(
+    v: Visitor<P>,
+    stmts: Stmt[],
+    ...p: P
+) {
+    for (const stmt of stmts) {
+        visitStmt(v, stmt, ...p);
+    }
+}
+
+export function visitStmt<
+    P extends PM = [],
+>(
+    v: Visitor<P>,
+    stmt: Stmt,
+    ...p: P
+) {
+    switch (stmt.kind.tag) {
+        case "error":
+            if (v.visitErrorStmt?.(stmt, ...p) === "stop") return;
+            return;
+        case "mod_block":
+            if (v.visitModBlockStmt?.(stmt, stmt.kind, ...p) === "stop") return;
+            visitStmts(v, stmt.kind.stmts, ...p);
+            return;
+        case "mod_file":
+            if (v.visitModFileStmt?.(stmt, stmt.kind, ...p) === "stop") return;
+            return;
+    }
+    exhausted(stmt.kind);
+}
diff --git a/compiler/ctx.ts b/compiler/ctx.ts
new file mode 100644
index 0000000..5d0267b
--- /dev/null
+++ b/compiler/ctx.ts
@@ -0,0 +1,107 @@
+import * as ast from "./ast/mod.ts";
+import { Diag } from "./diagnostics.ts";
+
+export class Ctx {
+    private fileIds = new Ids();
+    private files = new Map<Id<File>, FileInfo>();
+
+    private diags: Diag[] = [];
+
+    public fileHasChildWithIdent(file: File, childIdent: string): boolean {
+        return this.files.get(id(file))!
+            .subFiles.has(childIdent);
+    }
+
+    public addFile(
+        ident: string,
+        absPath: string,
+        relPath: string,
+        superFile: File | undefined,
+        text: string,
+    ): File {
+        const file = this.fileIds.nextThenStep();
+        this.files.set(id(file), {
+            ident,
+            absPath,
+            relPath,
+            superFile,
+            subFiles: new Map(),
+            text,
+        });
+        if (superFile) {
+            this.files.get(id(superFile))!
+                .subFiles.set(ident, file);
+        }
+        return file;
+    }
+
+    public addFileAst(file: File, ast: ast.File) {
+        this.files.get(id(file))!.ast = ast;
+    }
+
+    public fileInfo(file: File): FileInfo {
+        this.files.get(id(file))!;
+    }
+
+    public reportFatal(file: File, msg: string) {
+        console.error(`fatal: ${msg}`);
+        this.reportImmediately(file, msg);
+    }
+
+    public enableReportImmediately = false;
+    public enableStacktrace = false;
+    private reportImmediately(file: File, msg: string) {
+        if (!this.enableReportImmediately) {
+            return;
+        }
+
+        if (!this.enableStacktrace) {
+            return;
+        }
+        class StackTracer extends Error {
+            constructor() {
+                super("StackTracer");
+            }
+        }
+        try {
+            //throw new ReportNotAnError();
+        } catch (error) {
+            if (!(error instanceof StackTracer)) {
+                throw error;
+            }
+            console.log(
+                error.stack?.replace(
+                    "Error: StackTracer",
+                    "Stack trace:",
+                ) ??
+                    error,
+            );
+        }
+    }
+}
+
+export type File = IdBase;
+
+export type FileInfo = {
+    ident: string;
+    absPath: string;
+    relPath: string;
+    superFile?: File;
+    subFiles: Map<string, File>;
+    text: string;
+    ast?: ast.File;
+};
+
+export type IdBase = { id: number };
+
+export type Id<IdType extends IdBase> = IdType["id"];
+export const id = <IdType extends IdBase>(id: IdType): Id<IdType> => id.id;
+
+export class Ids<IdType extends IdBase> {
+    private next = 0;
+    public nextThenStep(): IdType {
+        const id = this.next;
+        this.next += 1;
+        return { id } as IdType;
+    }
+}
diff --git a/compiler/diagnostics.ts b/compiler/diagnostics.ts
new file mode 100644
index 0000000..080c030
--- /dev/null
+++ b/compiler/diagnostics.ts
@@ -0,0 +1,43 @@
+import { Ctx, File } from "./ctx.ts";
+
+export type Span = {
+    begin: Pos;
+    end: Pos;
+};
+
+export type Pos = {
+    idx: number;
+    line: number;
+    col: number;
+};
+
+export type Diag = {
+    severity: "fatal" | "error" | "warning" | "info";
+    origin?: string;
+    msg: string;
+    file?: File;
+    span?: Span;
+    pos?: Pos;
+};
+
+export function prettyPrintDiag(ctx: Ctx, diag: Diag) {
+    const { severity, msg } = diag;
+    const origin = diag.origin ? `${diag.origin}: ` : "";
+    console.error(`${origin}${severity}: ${msg}`);
+    if (diag.file && (diag.span || diag.pos)) {
+        const { absPath: path, text } = ctx.fileInfo(diag.file);
+        const { line, col } = diag.span?.begin ?? diag.pos!;
+        console.error(`    at ./${path}:${line}:${col}`);
+        if (diag.span) {
+            let begin = diag.span.begin.idx;
+            while (begin >= 0 && text[begin - 1] != "\n") {
+                begin -= 1;
+            }
+            let end = diag.span.end.idx;
+            while (end < text.length && text[end + 1] != "\n") {
+                end += 1;
+            }
+        } else if (diag.pos) {
+        }
+    }
+}
diff --git a/compiler/main.ts b/compiler/main.ts
index c87de99..c0b7153 100644
--- a/compiler/main.ts
+++ b/compiler/main.ts
@@ -1,14 +1,20 @@
-type Pack = {
+import * as path from "jsr:@std/path";
+import { Parser } from "./parser/parser.ts";
+import * as ast from "./ast/mod.ts";
+import { Ctx } from "./ctx.ts";
+import { File } from "./ctx.ts";
+
+export type Pack = {
     rootMod: Mod;
 };
 
-type Mod = null;
+export type Mod = null;
 
-interface PackEmitter {
+export interface PackEmitter {
     emit(pack: Pack): void;
 }
 
-class PackCompiler {
+export class PackCompiler {
     public constructor(
         private entryFilePath: string,
         private emitter: PackEmitter,
@@ -18,5 +24,53 @@ class PackCompiler {
     }
 }
 
-class ModResolver {
+type _P = { file: File };
+export class FileTreeCollector implements ast.Visitor<[_P]> {
+    private subFilePromise = Promise.resolve();
+
+    public constructor(
+        private ctx: Ctx,
+        private superFile: File | undefined,
+        private ident: string,
+        private absPath: string,
+        private relPath: string,
+    ) {}
+
+    public async collect(): Promise<void> {
+        const text = await Deno.readTextFile(this.absPath);
+        const file = this.ctx.addFile(
+            this.ident,
+            this.absPath,
+            this.relPath,
+            this.superFile,
+            text,
+        );
+        const fileAst = new Parser(file, text).parse();
+        this.ctx.addFileAst(file, fileAst);
+        ast.visitFile(this, fileAst, { file });
+        await this.subFilePromise;
+    }
+
+    visitModFileStmt(
+        _stmt: ast.Stmt,
+        kind: ast.ModFileStmt,
+        { file }: _P,
+    ): ast.VisitRes {
+        const { ident, filePath: relPath } = kind;
+        const absPath = path.join(path.dirname(this.absPath), relPath);
+        this.subFilePromise = this.subFilePromise
+            .then(() => {
+                if (this.ctx.fileHasChildWithIdent(file, ident)) {
+                }
+                return new FileTreeCollector(
+                    this.ctx,
+                    file,
+                    ident,
+                    absPath,
+                    relPath,
+                )
+                    .collect();
+            });
+        return "stop";
+    }
 }
diff --git a/compiler/parser/parser.ts b/compiler/parser/parser.ts
new file mode 100644
index 0000000..15186af
--- /dev/null
+++ b/compiler/parser/parser.ts
@@ -0,0 +1,14 @@
+import { File } from "../ast/ast.ts";
+import { File as CtxFile } from "../ctx.ts";
+import { todo } from "../util.ts";
+
+export class Parser {
+    public constructor(
+        private file: CtxFile,
+        private text: string,
+    ) {}
+
+    public parse(): File {
+        return todo();
+    }
+}
diff --git a/compiler/util.ts b/compiler/util.ts
new file mode 100644
index 0000000..7b8b9bf
--- /dev/null
+++ b/compiler/util.ts
@@ -0,0 +1,9 @@
+export function todo<T>(msg?: string): T {
+    class NotImplemented extends Error {}
+    throw new NotImplemented(msg);
+}
+
+export function exhausted(_: never) {
+    class Unexhausted extends Error {}
+    throw new Unexhausted();
+}