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Selector

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Type Definition

type SelectorType = Uint8Array & {
  readonly [brand]: "Selector";
  readonly length: 4;
};

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Creating Selectors

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From Signature String

import * as Selector from '@tevm/primitives';

const transferSelector = Selector.fromSignature('transfer(address,uint256)');
console.log(transferSelector.toHex()); // '0xa9059cbb'

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From Hex String

const selector = Selector.fromHex('0xa9059cbb');

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From Bytes

const bytes = new Uint8Array([0xa9, 0x05, 0x9c, 0xbb]);
const selector = Selector.from(bytes);

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Operations

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Convert to Hex

const hex = selector.toHex();
// '0xa9059cbb'

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Compare Selectors

const sel1 = Selector.fromSignature('transfer(address,uint256)');
const sel2 = Selector.fromSignature('approve(address,uint256)');
const equal = sel1.equals(sel2); // false

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Common Selectors

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ERC-20

• transfer(address,uint256) → 0xa9059cbb
• approve(address,uint256) → 0x095ea7b3
• balanceOf(address) → 0x70a08231
• totalSupply() → 0x18160ddd

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ERC-721

• transferFrom(address,address,uint256) → 0x23b872dd
• safeTransferFrom(address,address,uint256) → 0x42842e0e
• ownerOf(uint256) → 0x6352211e

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Signature Format

• transfer(address,uint256)
• swap(uint256,uint256,address,bytes)
• execute((address,uint256,bytes)[])

• transfer(address, uint256) (has spaces)
• transfer(address,uint) (should be uint256)
• Transfer(address,uint256) (wrong capitalization)

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How Selectors Work

1. Function Signature: Start with the canonical function signature
2. Hash: Compute keccak256(signature)
3. Truncate: Take the first 4 bytes

transfer(address,uint256)
  ↓ keccak256
0xa9059cbb2fead7aba11f5e0a11af...
  ↓ slice(0, 4)
0xa9059cbb

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Selector Collisions

1. One function will override the other in the contract
2. The EVM will use whichever function is defined first
3. This is considered a critical contract bug

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Usage in Contracts

1. Function Calls: First 4 bytes of transaction data identify the function
2. Low-level Calls: address.call(abi.encodeWithSelector(selector, args))
3. Function Routing: Contracts use selectors to route calls to the correct function

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API Reference

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Constructors

• from(value: SelectorLike): SelectorType - Create from various inputs
• fromHex(hex: string): SelectorType - Create from hex string
• fromSignature(signature: string): SelectorType - Compute from function signature

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Operations

• toHex(selector: SelectorType): string - Convert to hex string
• equals(a: SelectorType, b: SelectorType): boolean - Compare selectors

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See Also

• Selector (Effect) - Effect.ts integration with Schema validation
• FunctionSignature - Extended selector with metadata
• EventSignature - 32-byte event topic
• ErrorSignature - 4-byte error selector
• ABI Encoding - Encoding function calls