Writing scripts

An evs script is an ordinary TypeScript value. You build one with evscript, giving it a header — { name, args }, plus optional declared errors — and a builder callback that records what the script does. Compiling it produces EVM runtime bytecode plus a literal-typed ABI; executing it is a single eth_call.

Anatomy of a script

import { evscript, t } from '@maxencerb/evs';
import { erc20Abi } from 'viem';

const tokenMeta = evscript(
  // 1. header: the script name + an ordered list of argument types
  { name: 'tokenMeta', args: [t.address, t.address] },
  // 2. builder callback: args arrive as positional params after `s`
  (s, token, owner) => {
    const symbol = s.read({ address: token, abi: erc20Abi, functionName: 'symbol' });
    const decimals = s.read({ address: token, abi: erc20Abi, functionName: 'decimals' });
    const balance = s.read({
      address: token,
      abi: erc20Abi,
      functionName: 'balanceOf',
      args: [owner],
    });
    // 3. the callback must return s.return(...) — it defines the script's named outputs
    return s.return({ symbol, decimals, balance });
  },
);

const compiled = tokenMeta.compile(); // bytecode + literal-typed ABI, ready for viem

Three parts matter:

• The header { name, args } declares the script’s ABI surface (an optional errors: [...] adds custom errors for s.throw). name becomes the function name you pass to viem’s readContract; args is an ordered list of t.* types (a lone type or a readonly list). It is optional — a zero-arg script omits it.
• The builder callback receives s (a ScriptBuilder) followed by one positional handle per declared argument. It runs exactly once, on your machine — never on-chain.
• The callback must return s.return(...); the record you pass defines the script’s outputs.

evscript returns an EvsScript value with five members: name, ir (frozen, JSON-serializable), abi (the literal-typed ABI — it exists before you compile), errors (the declared t.error values, [] when none), and compile(options?), which is sugar for the standalone compile(script, options?). Exact signatures are in the evscript reference.

evscript uses const type parameters, so you never write as const on the header or the return record — inference is literal by default.

The header

• name must be a non-empty identifier (/^[A-Za-z_]\w*$/) other than __proto__, like every name evs accepts (args, struct fields, return keys, errors).
• args is a single t.* type or a readonly list of them (a lone type is sugar for a one-element list). It is optional. Declaration order is binding: it is the type-level order, the runtime encode order, and the ABI inputs order all at once. Call sites stay positional, viem style — args: [token, owner]. Each arg is auto-named arg{i} in the ABI (positional labels; viem infers args positionally).
• Valid argument types: any word type, string, bytes, a struct/tuple (t.struct/t.tuple), or an array of any of those up to four levels deep, dynamic or fixed-size (uint256[], tuple[], tuple[][], uint256[][][], string[][], uint256[2], t.array(Position, 3)). An array nested deeper than four levels throws a recording-time EvsTypeError (code UNSUPPORTED_V0). See values and types.
• Header validation is immediate: invalid identifiers and unknown type shapes throw EvsTypeError before anything records.

Reading arguments: positional params

Arguments arrive as positional parameters after s — there is no s.args. With the header above, the callback is (s, token, owner) => … where token is Expr<'address'> and owner is Expr<'address'>. A scalar arg is an Expr; a struct/tuple arg is a Tuple handle.

These are not values — they are typed handles standing in for the values that calldata will carry at run time. Values and types explains what you can and cannot do with a handle.

Returning values: s.return

• Call it exactly once, unconditionally, and return its result from the callback. Calling it inside s.if/s.while, twice, or not at all is a recording-time error (EvsScopeError/EvsTypeError).
• The record’s keys become the named components of the script’s single tuple output. viem consumers receive a plain object with the same keys — { symbol: 'DAI', decimals: 18, balance: 123n }-shaped, fully typed.
• The record needs at least one key. An empty record would return 0x, which viem reports as “returned no data”, so s.return({}) is rejected (a type error, and ABI_SHAPE at recording). A script that only guards (reverts or succeeds) returns a flag such as { ok: s.lit(t.bool, true) }.
• Every value is an Expr, a Tuple handle, or a MutArray handle — all returned directly (a struct flows out as an object, an array as a list). Read a cell with .get(); .expr() on a Tuple/MutArray is the equivalent optional form (the bare handle works too — see control flow).
• s.return seals the recorder: any builder call after it throws EvsScopeError (RECORDING_CLOSED).

Tip

Need a conditional result? Declare a cell with s.let, assign it inside s.if branches, and return cell.get() once at the end. See control flow.

Two timelines: recording time vs run time

This is the one mental model the whole library hangs on. The builder callback runs once, at recording time, in your JS process. Each builder call appends a statement to the script’s IR and hands back a typed placeholder (Expr) — s.read performs no call, x.add(1n) adds nothing. The compiled bytecode then replays the recorded statements at run time, inside eth_call on the node.

recording time (your machine)              run time (the node, inside eth_call)
─────────────────────────────              ────────────────────────────────────
evscript(header, callback)
  └─ callback runs ONCE, records IR  ──►   compiled bytecode executes
       s.read({...})  → Expr handle          STATICCALL token.symbol() → real bytes
       x.add(1n)      → Expr handle          checked ADD → real word
       s.return({...})                       ABI-encode outputs → returndata

Runs at recording time (TS)	Runs on-chain (compiled)
the builder callback, exactly once	the recorded statements, in recorded order
JS if/for over host values (unrolled, specialized)	s.if/s.while/s.for over runtime values
literal validation and folding, ABI resolution, selectors	checked arithmetic, sub-calls, decoding
s.fn body (once, at definition)	the subroutine, once per recorded call
loop-condition thunks, recorded once into the header	the header, once per iteration

The practical consequence: plain JS control flow is a metaprogramming tool. A JS loop over a host array runs at recording time and unrolls — the compiled script contains one copy of the body per iteration, and no loop:

import { evscript, t } from '@maxencerb/evs';
import { erc20Abi } from 'viem';

const TOKENS = [
  '0x6B175474E89094C44Da98b954EedeAC495271d0F', // DAI
  '0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2', // WETH
] as const;

const watchlist = evscript({ name: 'watchlist', args: [t.address] }, (s, owner) => {
  const out = s.newArray(t.uint256, TOKENS.length);
  let i = 0n;
  // host loop over a host array: runs at recording time and unrolls —
  // the compiled script contains one balanceOf call per token, no loop
  for (const token of TOKENS) {
    const bal = s.read({
      address: token,
      abi: erc20Abi,
      functionName: 'balanceOf',
      args: [owner],
    });
    out.set(i, bal);
    i += 1n;
  }
  return s.return({ balances: out.expr() });
});

When the data is only known at run time — a runtime address[] argument, a value returned by a call — you need the combinators instead: s.if, s.while, s.for, s.select. See control flow and the token balances example.

Caution

Native JS if/for/&& cannot branch on EVM values. An Expr is an object, and objects are always truthy — if (someExpr) records the branch unconditionally and JS cannot trap it. See the staging misuse rules for what is trapped and what is not.

Finding where an error came from

Builder calls validate eagerly and throw synchronously from inside your evscript callback, so a recording-time error’s ordinary JavaScript stack trace already points at the offending line of your script. Compile-time diagnostics and runtime reverts name the construct and the site id involved instead; see errors and debugging for the full diagnostics story.

Next

• Values and types — what an Expr is and which literals coerce.
• Calls — s.read and s.tryRead in depth.
• Execution — running the compiled script with viem.