Value DSL

The Value DSL is Fascia's restricted expression language for defining computed values, transition guards, invariants, and data transformations. It is intentionally limited -- no function definitions, no loops, no side effects. Every expression is a pure computation that produces a result from existing data.

Value DSL expressions appear throughout Fascia specs:

• Entity invariants -- Business rules that must always hold true
• Status machine guards -- Conditions that must be met for a transition to occur
• Flow Transform nodes -- Data transformations between flow steps
• If/Switch conditions -- Branching logic in flow graphs
• Policy conditions -- Rules evaluated by the Risk Engine

Operators

Arithmetic

Operator	Description	Example
+	Addition	order.subtotal + order.tax
-	Subtraction	order.total - order.discount
*	Multiplication	item.price * item.quantity
/	Division	total / count
%	Modulo	index % 2

Comparison

Operator	Description	Example
==	Equal	order.status == "confirmed"
!=	Not equal	user.role != "guest"
>	Greater than	reservation.totalPrice > 0
<	Less than	now() < reservation.startDate
>=	Greater than or equal	account.balance >= withdrawal.amount
<=	Less than or equal	deposit <= totalPrice

Logical

Operator	Description	Example
&&	Logical AND	isActive && isVerified
||	Logical OR	isAdmin || isStaff
!	Logical NOT	!isDeleted

Member Access

Use dot notation to access fields on entities and nested objects:

reservation.totalPrice
reservation.customer.name
input.paymentMethod

Built-in Functions

String

Function	Signature	Description
concat	concat(a, b, ...)	Concatenate strings
length	length(str)	String length
substring	substring(str, start, end)	Extract substring
toLowerCase	toLowerCase(str)	Convert to lowercase
toUpperCase	toUpperCase(str)	Convert to uppercase
trim	trim(str)	Remove leading/trailing whitespace
startsWith	startsWith(str, prefix)	Check if string starts with prefix
endsWith	endsWith(str, suffix)	Check if string ends with suffix
contains	contains(str, search)	Check if string contains substring
replace	replace(str, search, replacement)	Replace first occurrence
split	split(str, delimiter)	Split string into list

Date

Function	Signature	Description
now	now()	Current date and time
addDays	addDays(date, days)	Add days to a date
addHours	addHours(date, hours)	Add hours to a date
diffDays	diffDays(date1, date2)	Difference in days between two dates
format	format(date, pattern)	Format date as string

Math

Function	Signature	Description
abs	abs(n)	Absolute value
ceil	ceil(n)	Round up
floor	floor(n)	Round down
round	round(n)	Round to nearest integer
min	min(a, b)	Smaller of two values
max	max(a, b)	Larger of two values

Aggregation

Function	Signature	Description
sum	sum(list, field)	Sum a field across a list
count	count(list)	Count items in a list
avg	avg(list, field)	Average a field across a list

Type

Function	Signature	Description
toString	toString(value)	Convert to string
toNumber	toNumber(value)	Convert to number
isNull	isNull(value)	Check if value is null
coalesce	coalesce(a, b)	Return first non-null value

Conditional

Function	Signature	Description
if	if(condition, thenValue, elseValue)	Conditional expression

Example Expressions

Calculate 10% of a total:

reservation.totalPrice * 0.1

Conditional display text:

if(order.status == "pending", "Waiting for confirmation", "Processed")

Duration in days:

diffDays(reservation.endDate, reservation.startDate)

Check a business rule:

reservation.depositAmount <= reservation.totalPrice && reservation.totalPrice > 0

Format a summary string:

concat("Order #", toString(order.id), " - ", toUpperCase(order.status))

Constraints

The Value DSL is deliberately restricted for safety and predictability:

Constraint	Limit
No function definitions	You cannot define custom functions
No loops or recursion	Iteration is not supported
No variable mutation	All expressions are pure; no assignment
No external calls	Cannot call APIs or services
No system calls	Cannot access the filesystem, network, or OS
Maximum expression depth	256 levels of nesting
Maximum AST nodes	1000 nodes per expression

These constraints ensure that every Value DSL expression terminates in bounded time with no side effects, making the runtime fully deterministic.