import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import { z } from 'zod';
import { readFile } from 'node:fs/promises';
import { runGxi, escapeSchemeString } from '../gxi.js';
const FORM_MARKER = 'GERBIL-MCP-FORM:';
const ERROR_MARKER = 'GERBIL-MCP-READ-ERROR:';
interface FormInfo {
index: number;
startLine: number;
endLine: number;
summary: string;
}
function buildReadFormsExpr(escaped: string): string {
// Scheme code that reads all forms from a string port, tracking line numbers.
// For each form, outputs: MARKER index \t startLine \t endLine \t summary
return [
'(let* ((src "' + escaped + '")',
' (port (open-input-string src))',
' (idx 0))',
' (let loop ()',
' (let ((start-line (input-port-line port)))',
' (with-catch',
' (lambda (e)',
' (let ((eline (input-port-line port))',
' (ecol (input-port-column port)))',
' (display "' + ERROR_MARKER + '")',
' (display eline) (display "\\t")',
' (display ecol) (display "\\t")',
' (display-exception e (current-output-port))',
' (newline)))',
' (lambda ()',
' (let ((form (read port)))',
' (if (eof-object? form)',
' (void)',
' (let ((end-line (input-port-line port))',
' (summary (cond',
' ((pair? form)',
' (let ((hd (car form)))',
' (if (symbol? hd)',
' (symbol->string hd)',
' "(...)")))',
' ((symbol? form) (symbol->string form))',
' ((string? form) "\\"...\\"" )',
' ((number? form) (number->string form))',
' ((boolean? form) (if form "#t" "#f"))',
' (else (object->string form)))))',
' (display "' + FORM_MARKER + '")',
' (display idx) (display "\\t")',
' (display start-line) (display "\\t")',
' (display end-line) (display "\\t")',
' (displayln summary)',
' (set! idx (+ idx 1))',
' (loop)))))))))',
].join(' ');
}
function parseFormLines(output: string): { forms: FormInfo[]; error: string | null } {
const forms: FormInfo[] = [];
let error: string | null = null;
for (const line of output.split('\n')) {
if (line.startsWith(FORM_MARKER)) {
const rest = line.slice(FORM_MARKER.length);
const parts = rest.split('\t');
if (parts.length >= 4) {
forms.push({
index: parseInt(parts[0], 10),
startLine: parseInt(parts[1], 10),
endLine: parseInt(parts[2], 10),
summary: parts[3],
});
}
} else if (line.startsWith(ERROR_MARKER)) {
const rest = line.slice(ERROR_MARKER.length);
const parts = rest.split('\t');
if (parts.length >= 3) {
error = `Reader error at line ${parts[0]}, col ${parts[1]}: ${parts.slice(2).join('\t').trim()}`;
} else {
error = `Reader error: ${rest.trim()}`;
}
}
}
return { forms, error };
}
export function registerReadFormsTool(server: McpServer): void {
server.registerTool(
'gerbil_read_forms',
{
title: 'Read Top-Level Forms',
description:
'Read a Gerbil source file using the actual Gerbil reader and list all top-level forms ' +
'with their index, start/end line numbers, and a summary (car of list or type). ' +
'On reader error, reports the error position plus any forms read before the error.',
inputSchema: {
file_path: z
.string()
.describe('Path to a Gerbil source file to read'),
},
},
async ({ file_path }) => {
let source: string;
try {
source = await readFile(file_path, 'utf-8');
} catch (err) {
const msg = err instanceof Error ? err.message : 'Unknown error';
return {
content: [{ type: 'text' as const, text: `Failed to read file: ${msg}` }],
isError: true,
};
}
if (!source.trim()) {
return {
content: [{ type: 'text' as const, text: 'File is empty — no forms to read.' }],
};
}
const escaped = escapeSchemeString(source);
const expr = buildReadFormsExpr(escaped);
const result = await runGxi([expr]);
if (result.timedOut) {
return {
content: [{ type: 'text' as const, text: 'Reader timed out.' }],
isError: true,
};
}
const { forms, error } = parseFormLines(result.stdout);
const sections: string[] = [`File: ${file_path}`, ''];
if (forms.length > 0) {
sections.push(`Forms (${forms.length}):`);
for (const f of forms) {
const lineRange =
f.startLine === f.endLine
? `L${f.startLine}`
: `L${f.startLine}-${f.endLine}`;
sections.push(` [${f.index}] ${lineRange}: (${f.summary} ...)`);
}
}
if (error) {
sections.push('');
sections.push(error);
}
if (forms.length === 0 && !error) {
sections.push('No forms found.');
}
const hasError = !!error;
return {
content: [{ type: 'text' as const, text: sections.join('\n') }],
isError: hasError,
};
},
);
}
