write-a0

name: write-a0 description: This skill should be used when the user asks to "write an A0 program", "create an A0 script", "generate A0 code", "write a .a0 file", "A0 syntax", "A0 example", "how to write A0", "A0 language", or needs to produce any new A0 source code. Provides the complete syntax, type system, tool signatures, stdlib, and idiomatic patterns needed to author correct A0 programs.

Writing A0 Programs

A0 is a line-oriented scripting language for automation agents. Programs are sequences of statements producing a JSON result. Every program must end with return <expr> (any expression: record, literal, variable, arithmetic, list, etc.).

Core Syntax

# Comments start with #
cap { fs.read: true }        # capability declaration (top of file)
let name = expr              # bind a value
expr -> name                 # bind result of expression statement
return expr                  # required, must be last statement (any expression)

Types

Primitives: int float bool str null — literals: 42 3.14 true false null "hello"

Records: { key: value, nested: { a: 1 } } — keys may be dotted: { fs.read: true }. Spread syntax: { ...base, extra: 42 } merges base into the new record; later keys override earlier ones. Spreading a non-record produces E_TYPE.

Lists: [1, 2, "three"]

Strings are double-quoted with JSON escapes (\", \\, \n, \t).

Punctuation: ( ) for grouping expressions (controls operator precedence).

Arithmetic and Comparison Operators

Binary operators on numeric values, usable anywhere an expression is expected. AST node type: BinaryExpr.

Arithmetic Operators

let total = a + b
let diff = a - b
let product = a * b
let ratio = a / b
let remainder = a % b

Precedence follows standard math: *, /, % bind tighter than +, -. Use parentheses to override: let x = (a + b) * c.

Operands must be numbers (int or float). Using arithmetic on non-numbers produces E_TYPE (exit 4). Division or modulo by zero also produces E_TYPE.

Exception: string concatenation with +. The + operator also works on strings: "hello" + " world" produces "hello world". Both operands must be the same type (both strings or both numbers). Mixing types (e.g., "hello" + 1) produces E_TYPE.

Comparison Operators

let bigger = a > b
let ok = a <= threshold
let same = a == b
let diff = a != b

Full set: >, <, >=, <=, ==, !=. Return bool. Work on numbers and strings (lexicographic comparison for strings). Comparing incompatible types produces E_TYPE.

Comparison operators have lower precedence than arithmetic, so a + 1 > b means (a + 1) > b.

Unary Minus

Negate a numeric value. AST node type: UnaryExpr.

let neg = -x
let result = -(a + b)

Using unary minus on a non-number produces E_TYPE.

Capabilities

Declare required capabilities at the top of the file. Execution fails before any side-effect if the host policy denies them.

cap { fs.read: true, http.get: true }

Valid capabilities: fs.read, fs.write, http.get, sh.exec

Only declare capabilities the program actually uses.

Tools — Read vs Effect

Read-only tools use call?. Effectful tools use do. Using call? on an effect tool is a validation error (E_CALL_EFFECT, exit 2) caught by a0 check.

Tool arguments are always records { ... }, never positional. For full argument/return schemas, consult references/tool-signatures.md.

call? fs.read { path: "data.json" } -> content
do fs.write { path: "out.json", data: result, format: "json" } -> artifact

Stdlib Functions

Pure functions called like name { args }. No capability needed.

Data Functions

Predicate Functions

Predicates use A0 truthiness: false, null, 0, and "" are falsy; everything else is truthy.

List Operations

Math Operations

String Operations

Record Operations

let parsed = parse.json { in: raw_string }
let val = get { in: record, path: "nested.key[0]" }
let same = eq { a: 1, b: 1 }
let has_key = contains { in: record, value: "name" }
let count = len { in: items }
let nums = range { from: 1, to: 5 }
let sorted = sort { in: items, by: "name" }
let k = keys { in: record }
let full = str.concat { parts: ["hello", " ", "world"] }
let safe = coalesce { in: maybe_null, default: "fallback" }
let t = typeof { in: 42 }                        # "number"
let names = pluck { in: users, key: "name" }
let flat_list = flat { in: [[1, 2], [3]] }        # [1, 2, 3]
let pairs = entries { in: { a: 1 } }             # [{ key: "a", value: 1 }]
let path = str.template { in: "pkg/{name}", vars: { name: "core" } }

Evidence — assert & check

Both take { that: bool, msg: str } and produce evidence records in the trace.

• assert — Fatal. Halts execution immediately on failure (exit 5, E_ASSERT). Use for invariants that MUST hold — the program cannot continue meaningfully if these fail.
• check — Non-fatal. Records evidence (ok/fail) and continues execution. If ANY check fails, the program still completes but the runner returns exit 5 after execution finishes. Use for validations the agent should know about but that should not prevent the program from finishing.

Use predicate functions to produce meaningful boolean values:

let same = eq { a: actual, b: expected }
assert { that: same, msg: "values match" }

let has_name = contains { in: record, value: "name" }
check { that: has_name, msg: "record has name field" }

You can also use assert { that: true, msg: "..." } as an evidence marker to document that a step completed, or check { that: true, msg: "..." } to record non-fatal evidence.

Control Flow (v0.3)

if — Conditional expression

Record-style (simple value expressions) with lazy evaluation (only the taken branch evaluates):

let msg = if { cond: ok, then: "yes", else: "no" }

Block-style (statement bodies with return) — parenthesized condition, both branches required:

let result = if (ok) {
  let msg = "success"
  return { status: msg }
} else {
  let msg = "failure"
  return { status: msg }
}

Block if/else bodies work like fn or for bodies: they can contain let bindings, tool calls (call?/do), stdlib calls, and must end with return. Both the if and else branches are required. The condition uses A0 truthiness.

Uses A0 truthiness: false, null, 0, "" are falsy; everything else truthy.

for — List iteration

Iterates a list, producing a list of results. Each iteration runs in its own scope.

let results = for { in: items, as: "item" } {
  let processed = parse.json { in: item }
  return { data: processed }
}

Body must end with return. Budget-aware via maxIterations. The loop variable (item) is scoped to the body. Tool calls (call?/do) work inside for bodies.

map — Higher-order list transformation

Apply a user-defined function to every element of a list, collecting results. The function must be defined with fn before use and referenced by name as a string.

fn double { x } {
  return { val: x * 2 }
}
let doubled = map { in: [1, 2, 3], fn: "double" }
# doubled == [{ val: 2 }, { val: 4 }, { val: 6 }]

Multi-param functions work with record items — params are destructured from record keys:

fn fullName { first, last } {
  let name = str.concat { parts: [first, " ", last] }
  return { name: name }
}
let names = map { in: users, fn: "fullName" }

Budget-aware via maxIterations (shared counter with for, filter (fn:), and reduce). Errors propagate immediately — no partial results.

reduce — Accumulate a list to a single value

Apply a 2-parameter user-defined function to accumulate a list into a single value. The function receives (accumulator, item) for each element.

fn addScore { acc, item } {
  let newTotal = acc.val + item.score
  return { val: newTotal }
}
let result = reduce { in: items, fn: "addScore", init: { val: 0 } }
# result.val contains the sum

The callback must accept exactly 2 parameters. Budget-aware via maxIterations (shared counter with for, map, filter, loop).

filter block — Inline predicate filtering (v0.5)

Filter a list with an inline predicate body. More concise than filter { fn: ... } when the predicate is simple.

let positives = filter { in: nums, as: "x" } {
  return x > 0
}

The body runs once per element. If the return value is truthy, the original item is kept. Uses the same truthiness unwrapping as filter { fn: ... } — if a record is returned, checks the first value; if a bare value is returned, checks it directly.

Backward compatible: filter { in: list, by: "key" } and filter { in: list, fn: "pred" } still work (no block = old behavior).

Budget-aware via maxIterations (shared counter with for, map, reduce, loop).

loop — Iterative convergence (v0.5)

Run a body a fixed number of times, threading state through each iteration. The body receives the current value and returns the next value.

let result = loop { in: 0, times: 5, as: "x" } {
  return x + 1
}
# result == 5

Complex state with records:

let result = loop { in: { sum: 0, count: 0 }, times: 3, as: "acc" } {
  return { sum: acc.sum + acc.count + 1, count: acc.count + 1 }
}

• in: — initial value (any type)
• times: — number of iterations (must be a non-negative integer, else E_TYPE)
• as: — binding name for current value in each iteration
• 0 iterations returns the in: value unchanged

Budget-aware via maxIterations (shared counter with for, map, filter, reduce).

fn — User-defined functions

Define before use. Called with record-style arguments. Direct recursion allowed. Functions can access variables from their defining scope (closure).

fn greet { name } {
  return { greeting: "hello", who: name }
}
let result = greet { name: "world" }

Parameters are destructured from the caller's record. Missing params default to null. Body must end with return.

Closure example — a function capturing an outer variable:

let threshold = 18
fn isAdult { item } {
  return { ok: item.age >= threshold }
}
let adults = filter { in: users, fn: "isAdult" }

The function isAdult captures threshold from the scope where it was defined.

match — ok/err discrimination

Discriminates records with ok or err keys. Two forms:

# Match on a variable (identPath)
let output = match result {
  ok { val } {
    return { data: val }
  }
  err { e } {
    return { error: e }
  }
}

# Match on an expression (parenthesized)
let output = match ({ ok: 42 }) {
  ok { v } {
    return { value: v }
  }
  err { e } {
    return { error: e }
  }
}

Subject must be a record with ok or err key. When matching on a variable, use match ident { ... }. When matching on an expression, wrap it in parentheses: match ( expr ) { ... }. Both arms must end with return.

try/catch — Error recovery

Catch runtime errors (tool failures, type errors, stdlib errors, etc.) without halting the program. The catch binding receives a { code, message } record.

let result = try {
  call? fs.read { path: "maybe-missing.txt" } -> content
  let parsed = parse.json { in: content }
  return { data: parsed }
} catch { e } {
  return { error: e.code, detail: e.message }
}

The try body executes normally. If any statement throws, control transfers to the catch body. The binding { e } uses the same syntax as match arm bindings. Both bodies must end with return.

Caught errors include: E_TOOL, E_TOOL_ARGS, E_FN, E_TYPE, E_PATH, E_FOR_NOT_LIST, E_MATCH_NOT_RECORD, E_MATCH_NO_ARM, and other runtime errors. Note: E_ASSERT (fatal assertion) is NOT catchable -- it always halts the program.

Budget

Declare resource limits with budget { ... } at the top of the file (before or after cap). Exceeding a limit stops execution with E_BUDGET (exit 4).

budget { timeMs: 30000, maxToolCalls: 10, maxBytesWritten: 1048576, maxIterations: 100 }

Only declare budget fields the program needs. Unknown fields produce E_UNKNOWN_BUDGET at validation time.

Property Access

Dot notation on bound variables: response.body, result.exitCode, data.items.

Program Rules

1. return <expr> is required and must be the last statement. Can return any expression (record, literal, variable, arithmetic, list, etc.).
2. Variables must be bound with let or -> before use.
3. No duplicate let bindings in the same scope.
4. call? for read-mode tools only; do for effect-mode tools only.
5. Tool and function args are always records { ... }.
6. Reserved words cannot be variable names: cap, let, return, do, assert, check, true, false, null, import, as, budget, if, else, for, fn, match, try, catch, filter, loop.
7. fn bodies, for bodies, match arms, filter block bodies, and loop bodies must each end with return.
8. fn must be defined before use (no hoisting).

Common Mistakes

Avoid these frequent errors:

• Forgetting return → E_NO_RETURN. Every program must end with return <expr>.
• return not last → E_RETURN_NOT_LAST. No statements after return.
• Using call? for an effect tool → E_CALL_EFFECT. Use do for fs.write and sh.exec.
• Using do for a read tool — allowed but unconventional. Prefer call? for fs.read and http.get to signal read-only intent.
• Undeclared capability → E_UNDECLARED_CAP. Declare the capability in cap { ... } for each tool used.
• Capability denied by policy → E_CAP_DENIED. Update the policy file or use --unsafe-allow-all.
• Budget exceeded → E_BUDGET. Increase the budget limit or reduce resource usage.
• Unknown budget field → E_UNKNOWN_BUDGET. Valid fields: timeMs, maxToolCalls, maxBytesWritten, maxIterations.
• Duplicate function name → E_FN_DUP. Each fn name must be unique.
• map with non-list in → E_TYPE. The in: value must be a list.
• map with non-string fn → E_TYPE. The fn: value must be a function name string.
• map with unknown function → E_UNKNOWN_FN. The named function must be defined with fn before the map call.
• reduce with non-2-param function → E_TYPE. The callback must accept exactly 2 parameters (accumulator, item).
• reduce with unknown function → E_UNKNOWN_FN. The named function must be defined with fn before the reduce call.
• filter with unknown function → E_UNKNOWN_FN. When using fn:, the named function must be defined with fn before the filter call.
• filter with neither by nor fn → E_FN. Must provide either by: (key name) or fn: (predicate function name).
• filter with both by and fn → E_FN. Provide exactly one of by: or fn:, not both.
• filter block on non-list → E_TYPE. The in: value must evaluate to a list.
• loop with non-integer times → E_TYPE. The times: value must be a non-negative integer (floats like 2.5 also rejected).
• loop with negative times → E_TYPE. Use 0 or positive integers.
• for on non-list → E_FOR_NOT_LIST. The in: value must evaluate to a list.
• match on non-record → E_MATCH_NOT_RECORD. The subject must be a record with ok or err key.
• match missing arm → E_MATCH_NO_ARM. Subject record must have ok or err key.
• Type error in expression → E_TYPE. Arithmetic on non-numbers, division by zero, comparing incompatible types, mixed types with + (e.g., "hello" + 1), or spreading a non-record.
• Missing else in block if → Parse error. Block if/else requires both branches.
• Reusing a variable name → E_DUP_BINDING. Each let name must be unique.
• Using a variable before binding → E_UNBOUND. Bind with let or -> first.
• Positional arguments → Parse error. Always use record syntax { key: value }.

Best Practices

Prefer these patterns to produce shorter, more robust programs:

1. Discover dynamically — Use fs.list to discover directories and files at runtime instead of hardcoding paths. Hardcoded paths break when the project structure changes and bloat the program with repetitive path literals. See Pattern 17.
2. Inherit with coalesce — Use coalesce { in: val, default: fallback } for null-defaulting instead of manual if/else fallback chains. It's one line vs. five, and preserves falsy values (0, false, "").
3. Compose with spread — Use { ...base, key: val } to extend records instead of manually repeating every field. This is especially valuable inside for loops when augmenting each item.
4. Compute, don't enumerate — Derive values algorithmically with keys, filter, pluck, math.max, sort { by: [...] } (multi-key) instead of writing out each case. If you find yourself listing items one-by-one, there's usually a stdlib function that computes the answer.
5. Define functions before use — fn declarations are not hoisted. Define helper functions before the first filter { fn: "..." }, map { fn: "..." }, or reduce { fn: "..." } call that references them.

Idiomatic Patterns

For complete patterns (HTTP+transform, shell exec, validation, data pipelines), see references/patterns.md.

Minimal program (pure data)

let data = { name: "example", version: 1 }
return { result: data }

HTTP fetch + transform

cap { http.get: true, fs.write: true }
call? http.get { url: "https://api.example.com/data" } -> resp
let body = parse.json { in: resp.body }
let title = get { in: body, path: "title" }
do fs.write { path: "out.json", data: { title: title }, format: "json" } -> artifact
return { artifact: artifact }

Shell command

cap { sh.exec: true }
do sh.exec { cmd: "echo hello", timeoutMs: 5000 } -> result
assert { that: true, msg: "command succeeded" }
return { stdout: result.stdout, exitCode: result.exitCode }

CLI Quick Reference

a0 check file.a0                     # validate syntax + semantics
a0 run file.a0                       # execute (deny-by-default)
a0 run file.a0 --unsafe-allow-all    # allow all caps (dev only)
a0 run file.a0 --trace t.jsonl       # emit JSONL trace
a0 run file.a0 --pretty              # human-readable error output
a0 fmt file.a0                       # canonical format to stdout
a0 fmt file.a0 --write               # format in place

Exit codes: 0 ok, 2 parse error, 3 capability denied, 4 runtime error, 5 assert/check failed. assert = fatal (halts immediately), check = non-fatal (records evidence, continues; exit 5 after run if any check failed).

Additional Resources

Reference Files

• references/tool-signatures.md — Full argument and return schemas for all built-in tools
• references/patterns.md — Idiomatic program patterns with complete working examples

Example Files

Working .a0 programs in examples/:

• examples/hello.a0 — Minimal pure-data program
• examples/fetch-transform.a0 — HTTP fetch, parse, write pattern
• examples/if-demo.a0 — Conditional branching with if
• examples/for-demo.a0 — List iteration with for
• examples/fn-demo.a0 — User-defined functions with fn
• examples/match-demo.a0 — ok/err discrimination with match
• examples/map-demo.a0 — Higher-order list transformation with map
• examples/bare-return-demo.a0 — Returning bare values (non-record expressions)
• examples/filter-block-demo.a0 — Inline filter block with predicate body
• examples/loop-demo.a0 — Iterative convergence with loop
• examples/list-packages.a0 — Dynamic package discovery with fs.list, for, sort, math.max, reduce