Superscript compiler

This small programming language has been designed to better illustrate the advanced scripting used in the Bitcoin contracts article.

Note that this compiler is made only for educational purposes and is highly experimental and not actively maintained. You are welcomed to fork and hack on it.

Both Superscript and Bitcoin Script are stack-oriented programming languages and that makes the simple compiler (scripts/bytecode.js) trivial.

As each byte of Bitcoin transaction makes the transaction fees bigger the full compiler is comprised of a set of optimizations (e.g. optimizing OP_0 OP_PICK into one OP_DUP). Most of them are generic (can be applied to any script) but some of them are specific to scripts used in the Bitcoin contracts article.

The language

The Superscript code consist of an optional assume statement followed by a number of assignments, conditionals or verification statements.

Assume statements

They do not generate any code but are ambient declarations about the state of stack before the script executes. Basically they describe the expected input script and are used by the compiler to resolve variable names to their positions on the stack.

assume signatureA, signatureB, secret

This statement expects one secret and two signatures to appear on the stack, secret being the top-most item.

Assignments

They range from the immutable bindings (let NAME = value) to mutable bindings (let mutable NAME = value) to modifications (NAME <- value). Modifications can work only on names bound to mutable names. Immutable bindings can be inlined.

let pubKeyA = '04d4bf4642f56fc7af0d2382e2cac34fa16ed3321633f91d06128f0e5c0d17479778cc1f2cc7e4a0c6f1e72d905532e8e127a031bb9794b3ef9b68b657f51cc691'
let signedByA = checkSig(signatureA, pubKeyA)

Conditional statements

They take an expression and if it is truthy execute the associated block (if (expr) { block }) optionally providing an alternative block (else) that is executed if the expression is not truthy.

let mutable num = sizeB + sizeC + sizeA - 96
if (num > 2) {
    num <- num - 3
}

Verification statements

These statements evaluate the expression and if it is not truthy abort the script execution. Verify can emit OP_VERIFY opcode or be glued during optimization with previous opcode (e.g. producing OP_EQUALVERIFY).

Verify statements at the end of the script are removed entirely (see dropLastVerify in scripts/optimizations.js). This is safe as Bitcoin script execution requires a truthy value on the top of the stack to succeed.

verify (secretKnown || signedByB) && signedByA

Function calls

Each function call produces an opcode derived from the function name (e.g. sha256(secret) produces OP_SHA256). Some opcodes have special behavior that influences the compiler (see scripts/builtin.js). Operators (such as ==) map to opcodes (see scripts/emitter.js).

For more technical details of Bitcoin Script execution see Script on Bitcoin Wiki.