Steel language reference

Table scripts use a fail-closed allowlist. The server macro-expands the program with Steel’s own parser and rejects every form or identifier that is not explicitly accepted.

Syntax and result

A script must be a non-empty S-expression beginning with (. Steel validates balanced syntax. Literals, comments, quoting, if, begin, and, or, let, lambdas, and immutable vectors are supported when their contents also pass the allowlist.

The last expression supplies the target value. A target must ultimately receive a string, number, integer, boolean, or correctly typed money result.

Database-aware functions

Function	Purpose
`steel_get_column name column`	Read one current-row or linked-row column
`steel_related_aggregate operation table column anchor`	Aggregate a related collection
`steel_related_count_rows table anchor`	Count related rows
`steel_related_exists table anchor`	Test for related rows
`has-var? name`	Test whether a key exists in the active row snapshot
`quantity-add link column amount`	Record a positive quantity-ledger contribution
`quantity-subtract link column amount`	Record a negative quantity-ledger contribution

All database names and operations must be string literals. These functions read request context prepared outside the VM; they do not issue unrestricted queries themselves.

Quantity contributions are explained in Quantity ledger.

Decimal operations

Source math forms are transformed to exact-decimal operations before the script is stored:

+  -  *  /  ^  **  pow  sqrt
>  <  =  >=  <=  min  max  abs  round
ln  log  log10  exp  sin  cos  tan

Direct decimal functions are also available:

to-decimal
decimal-add  decimal-sub  decimal-mul  decimal-div
decimal-abs  decimal-round  decimal-min  decimal-max
decimal-eq  decimal-gt  decimal-gte  decimal-lt  decimal-lte
decimal-zero  decimal-one  decimal-pi  decimal-e
decimal-exp  decimal-ln  decimal-log10  decimal-pow  decimal-sqrt
decimal-sin  decimal-cos  decimal-tan
decimal-compound  decimal-percentage  decimal-format
set-precision  clear-precision  get-precision

Per-script precision changes are cleared after execution and cannot leak into another request.

Columns with these kinds cannot participate in math:

BIGINT  TEXT  BOOLEAN
PHONE and phone component types
IBAN and IBAN component types
CREDIT_CARD
DATE  TIME  INSTANT  USER_DATETIME  RAW_DATETIME
DURATION  PERIOD  TIMESTAMPTZ  TIMESTAMP

Money operations

Money is a currency-aware runtime value. Use explicit functions:

money?  money-new
money-add  money-sub  money-mul  money-div
money-abs  money-amount
money-eq  money-gt  money-gte  money-lt  money-lte

Rules:

money-new requires an uppercase ISO currency and a decimal amount;
addition, subtraction, and comparison require matching currencies;
multiplication and division take a non-money decimal scalar;
money-amount explicitly extracts an amount for decimal operations;
every money-returning branch must return the same currency;
a MONEY target must receive its configured currency;
a non-money target cannot receive a money value.

(money-add
  (money-new "EUR" "10.00")
  (money-new "EUR" "2.50"))

See How money works for storage and rounding rules.

Pure helpers

Numbers and predicates

quotient  remainder  modulo  expt
exact->inexact  inexact->exact  floor  ceiling  truncate
number?  complex?  real?  rational?  int?  integer?  exact-integer?
float?  nan?  positive?  negative?  zero?  even?  odd?
exact?  inexact?  finite?  infinite?
boolean?  not  eq?  eqv?  equal?

Strings and characters

string?  string  string-append  to-string
string-length  utf8-length  substring  string-ref
string=?  string-ci=?  string<?  string-ci<?
string<=?  string-ci<=?  string>?  string-ci>?
string>=?  string-ci>=?
string->list  string->vector  string->bytes  string->number  string->int
number->string  int->string  string->symbol  symbol->string
string-upcase  string-downcase  string-foldcase
starts-with?  ends-with?  string-contains?  string-replace  string-push
make-string  split-whitespace  split-once  split-many  string-join
trim  trim-start  trim-end  trim-start-matches  trim-end-matches
char-upcase  char-downcase  char-foldcase  char-digit?  char-whitespace?
char->number  char->integer  integer->char  char=?  char-ci=?

Lists, immutable vectors, and hashes

list  list?  pair?  null?  car  cdr  cons  length  list-ref
append  reverse  member  memq  list-contains  second  third
take  list-tail  list-drop  try-list-ref  list-chunks  list->string
list->vector  empty?

immutable-vector  make-immutable-vector  vector-ref  vector-length
immutable-vector->list  immutable-vector->string  immutable-vector-copy
immutable-vector-append  immutable-vector-push  immutable-vector-rest
immutable-vector-take  immutable-vector-drop

hash  hash-ref  hash-try-get  hash-length  hash-contains?  hash-empty?
hash-insert  hash-remove  hash-clear  hash-union

The hash functions above return new values; mutation is not enabled.

Higher-order helpers

map  filter  foldl  fold  reduce  apply  for-each  range
first  rest  last  list-sort

Functions passed to a higher-order helper must themselves be allowed.

Target-column rules

Common supported targets include TEXT, BOOLEAN, INT, DECIMAL, and MONEY.

A script cannot target:

system columns: id, deleted, created_at, or row_revision;
a quantity-ledger-backed column;
an accounting-transfer source column;
BIGINT;
phone, IBAN, or credit-card types;
DATE, TIME, INSTANT, datetime, duration, period, timestamp, or timestamptz types.

BIGINT is also prohibited as a referenced input. Other non-math types may be read for supported value operations but cannot be passed through decimal arithmetic.

Rejected behavior

The following forms illustrate behavior that is not permitted:

steel_query_sql
define  define-syntax  define-values
set!  set-car!  set-cdr!  set-box!  box-set!
vector-set!  mutable-vector-set!  string-set!  bytes-set!
hash-set!  hash-remove!
eval  load  require  return

Unknown functions are rejected as well. Scripts cannot import code, evaluate generated code, change global bindings, mutate collections, access undeclared database data, or retain state between requests.