/**
* gerbil_splice_form — Remove a wrapper form, keeping selected children.
*
* Pure TypeScript, no subprocess.
*/
import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import { z } from 'zod';
import { readFile, writeFile } from 'node:fs/promises';
import { checkBalance } from './check-balance.js';
import { findFormAt } from './scheme-scanner.js';
export function registerSpliceFormTool(server: McpServer): void {
server.registerTool(
'gerbil_splice_form',
{
title: 'Splice Form',
description:
'Remove a wrapper form, keeping selected children (inverse of wrap). ' +
'E.g. splicing (when cond (do-x) (do-y)) with keep_children [2, 3] produces (do-x)\\n(do-y). ' +
'Child indices are 1-based (1=head token, 2=first arg, etc.). ' +
'Default: keep all children except the head (index 1). ' +
'Dry-run by default. Pure TypeScript — no subprocess.',
inputSchema: {
file_path: z.string().describe('Absolute path to the Gerbil source file'),
line: z
.number()
.int()
.describe('Line number (1-based) where the form to splice starts'),
keep_children: z
.array(z.number().int())
.optional()
.describe(
'Which children to keep (1-based indices). Default: all except index 1 (the head). ' +
'E.g. for (when cond body1 body2): 1=when, 2=cond, 3=body1, 4=body2.',
),
dry_run: z
.boolean()
.optional()
.describe(
'If true (default), only preview the change without writing. Set to false to apply.',
),
},
},
async ({ file_path, line: targetLine, keep_children, dry_run }) => {
const isDryRun = dry_run !== false;
// Read file
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,
};
}
// Find the form at the target line
const form = findFormAt(source, targetLine);
if (!form) {
return {
content: [
{
type: 'text' as const,
text: `No form found at or near line ${targetLine}.`,
},
],
isError: true,
};
}
const children = form.children;
if (children.length === 0) {
return {
content: [
{
type: 'text' as const,
text: `Form at line ${form.start.line} has no children to splice.`,
},
],
isError: true,
};
}
// Determine which children to keep (1-based indices)
let keepIndices: number[];
if (keep_children && keep_children.length > 0) {
keepIndices = keep_children;
} else {
// Default: all except head (index 1)
keepIndices = [];
for (let ci = 2; ci <= children.length; ci++) {
keepIndices.push(ci);
}
}
// Validate indices
for (const idx of keepIndices) {
if (idx < 1 || idx > children.length) {
return {
content: [
{
type: 'text' as const,
text: `Child index ${idx} is out of range (form has ${children.length} children).`,
},
],
isError: true,
};
}
}
if (keepIndices.length === 0) {
return {
content: [
{
type: 'text' as const,
text: `No children selected to keep. This would delete the entire form.`,
},
],
isError: true,
};
}
// Extract kept children text
const keptTexts: string[] = [];
for (const idx of keepIndices) {
keptTexts.push(children[idx - 1].text);
}
// Detect indentation of the original form
const sourceLines = source.split('\n');
const formLineText = sourceLines[form.start.line - 1] || '';
const leadingMatch = formLineText.match(/^(\s*)/);
const baseIndent = leadingMatch ? leadingMatch[1] : '';
// Build spliced text with proper indentation
const splicedLines: string[] = [];
for (const text of keptTexts) {
const textLines = text.split('\n');
for (let li = 0; li < textLines.length; li++) {
const trimmed = textLines[li].replace(/^\s*/, '');
if (trimmed.length === 0 && li > 0) {
splicedLines.push('');
} else if (li === 0) {
splicedLines.push(baseIndent + trimmed);
} else {
splicedLines.push(baseIndent + ' ' + trimmed);
}
}
}
const splicedText = splicedLines.join('\n');
// Build new file content by replacing the form span
const before = source.slice(0, form.start.offset);
const after = source.slice(form.end.offset);
const newSource = before + splicedText + after;
// Validate balance
const originalBalance = checkBalance(source);
const newBalance = checkBalance(newSource);
if (originalBalance.ok && !newBalance.ok) {
const errorDetails = newBalance.errors
.map((e) => ` ${e.kind}: '${e.char}' at line ${e.line}, col ${e.col}`)
.join('\n');
return {
content: [
{
type: 'text' as const,
text:
`REJECTED: Splicing would break delimiter balance.\n\n` +
`Balance errors:\n${errorDetails}\n\n` +
`The file was NOT modified.`,
},
],
isError: true,
};
}
// Format children summary for display
const childSummary = children
.map((c, i) => ` ${i + 1}: ${c.kind} "${c.text.length > 40 ? c.text.slice(0, 40) + '...' : c.text}"`)
.join('\n');
if (isDryRun) {
const lines: string[] = [];
lines.push(
`Dry run — splice form at line ${form.start.line} (${form.opener}${form.headToken || ''}...):`,
);
lines.push('');
lines.push('Children:');
lines.push(childSummary);
lines.push('');
lines.push(`Keeping indices: [${keepIndices.join(', ')}]`);
lines.push('');
lines.push('--- result ---');
lines.push(splicedText);
lines.push('---');
lines.push('');
lines.push(`Balance: OK (${newBalance.topLevelForms} top-level forms)`);
lines.push('');
lines.push('Set dry_run to false to apply this change.');
return {
content: [{ type: 'text' as const, text: lines.join('\n') }],
};
}
// Apply the change
await writeFile(file_path, newSource, 'utf-8');
return {
content: [
{
type: 'text' as const,
text:
`Spliced form at line ${form.start.line} in ${file_path}\n` +
`Kept children: [${keepIndices.join(', ')}]\n` +
`Balance: OK (${newBalance.topLevelForms} top-level forms)`,
},
],
};
},
);
}
