Skip to main content

String Interpolation

Build dynamic strings by embedding expressions directly inside string literals using ${expression} syntax.

Use Case​

You need to format user-facing messages that include computed values, without manual concatenation.

The Pipeline​

# string-interpolation.cst

@example("Alice")
in userName: String

@example(28)
in userAge: Int

@example(1500)
in accountBalance: Int

# Simple variable interpolation
greeting = "Hello, ${userName}!"

# Arithmetic expressions in interpolation
ageNextYear = "Next year you will be ${userAge + 1} years old."

# Multiple interpolations in one string
summary = "User ${userName} is ${userAge} years old."

# Escape sequences: \n for newline, \t for tab, \$ for literal dollar sign
formatted = "Account Summary:\n\tName: ${userName}\n\tBalance: \$${accountBalance}"

# Conditional message using interpolation with branch
balanceStatus = branch {
  accountBalance > 1000 -> "healthy",
  accountBalance > 0 -> "low",
  otherwise -> "overdrawn"
}
statusMessage = "Your account status is: ${balanceStatus}"

# Combining with other operations
upperName = Uppercase(userName)
formalGreeting = "Dear ${upperName}, welcome to our service!"

out greeting
out ageNextYear
out summary
out formatted
out statusMessage
out formalGreeting

Explanation​

FeatureSyntaxExample
Variable${varName}"Hello, ${userName}!"
Arithmetic${expr + expr}"Age: ${userAge + 1}"
Newline\n"Line1\nLine2"
Tab\t"Col1\tCol2"
Literal $\$"Price: \$${amount}"

Interpolation expressions are evaluated at runtime. Any expression valid in an assignment can appear inside ${}.

tip

Keep interpolation expressions simple. For complex logic, compute the value in a separate variable first, then interpolate it.

Running the Example​

Input​

{
  "userName": "Alice",
  "userAge": 28,
  "accountBalance": 1500
}

Expected Output​

{
  "greeting": "Hello, Alice!",
  "ageNextYear": "Next year you will be 29 years old.",
  "summary": "User Alice is 28 years old.",
  "formatted": "Account Summary:\n\tName: Alice\n\tBalance: $1500",
  "statusMessage": "Your account status is: healthy",
  "formalGreeting": "Dear ALICE, welcome to our service!"
}

Variations​

Template with multiple fields​

type User = { name: String, role: String }
in user: User

welcome = "Welcome ${user.name}, you are logged in as ${user.role}."

out welcome
warning

Don't forget to escape $ with \$ when you need a literal dollar sign. "Price: $100" will fail; use "Price: \$100" instead.

Best Practices​

  1. Use interpolation instead of concat — "Hello, ${name}!" is clearer than concat("Hello, ", concat(name, "!"))
  2. Escape $ when needed — use \$ for literal dollar signs in financial output
  3. Keep expressions simple — complex logic should be in a separate variable, then interpolated