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Branch Expressions

Use branch { } for clean multi-way conditionals, replacing nested if/else chains.

Use Case​

You need to classify a value into one of several categories based on conditions.

The Pipeline​

# branch-expressions.cst

@example("high")
in priority: String

@example(85)
in score: Int

@example("technical")
in category: String

# Basic branch: string matching
processingTier = branch {
  priority == "high" -> "fast-path",
  priority == "medium" -> "standard",
  otherwise -> "batch"
}

# Numeric ranges (first match wins)
grade = branch {
  score >= 90 -> "A",
  score >= 80 -> "B",
  score >= 70 -> "C",
  score >= 60 -> "D",
  otherwise -> "F"
}

# Complex boolean conditions
isUrgent = priority == "high"
isHighScore = score >= 75

actionPlan = branch {
  isUrgent and isHighScore -> "immediate-review",
  isUrgent and not isHighScore -> "escalate",
  isHighScore -> "queue-review",
  otherwise -> "standard-process"
}

# Category routing
department = branch {
  category == "sales" -> "Sales Team",
  category == "support" -> "Customer Service",
  category == "technical" -> "Engineering",
  category == "billing" -> "Finance",
  otherwise -> "General Inquiries"
}

out processingTier
out grade
out actionPlan
out department

Explanation​

PartSyntaxPurpose
Branch keywordbranch { ... }Opens a multi-way conditional
Armcondition -> resultEvaluated in order; first true condition wins
Otherwiseotherwise -> defaultRequired fallback if no condition matches

Branch expressions are expressions — they produce a value that can be assigned to a variable, interpolated into a string, or passed to a module.

warning

Conditions are evaluated top-to-bottom. The first matching condition wins, so order from most specific to least specific.

Running the Example​

Input​

{
  "priority": "high",
  "score": 85,
  "category": "technical"
}

Expected Output​

{
  "processingTier": "fast-path",
  "grade": "B",
  "actionPlan": "immediate-review",
  "department": "Engineering"
}

Variations​

Branch vs if/else​

in score: Int

# Branch (preferred for 3+ cases)
grade_branch = branch {
  score >= 90 -> "A",
  score >= 80 -> "B",
  otherwise -> "C or below"
}

# If/else (fine for 2 cases)
pass_fail = if (score >= 60) "Pass" else "Fail"

out grade_branch
out pass_fail
tip

The otherwise clause is required. The compiler will reject a branch expression without a fallback, ensuring every input produces a result.

Best Practices​

  1. Use branch for 3+ conditions — if/else is fine for binary choices
  2. Order conditions from most specific to least — conditions are evaluated top to bottom
  3. Always include otherwise — the compiler requires a fallback case