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Pattern Matching

Prism supports Rust-style match expressions for structural matching with bindings, guards, and confidence thresholds. match is an expression, so it always returns a value.

Basic Match​

let result = match value {
  0 => "zero",
  1 => "one",
  _ => "other"
}

Bindings and Wildcards​

let label = match score {
  x if x > 90 => "A",
  _ => "not A"
}

Use _ to ignore a value without binding.

Array Patterns​

let outcome = match [1, 2, 3] {
  [a, b, ...rest] => rest,
  _ => []
}

Object Patterns​

let message = match {type: "error", message: "fail"} {
  {type: "error", message} => message,
  _ => "ok"
}

Nested Patterns​

let status = match {meta: {ok: true}} {
  {meta: {ok: true}} => "ready",
  _ => "pending"
}

Guards​

let bucket = match value {
  x if x > 10 => "large",
  x if x > 0 => "small",
  _ => "zero"
}

Confidence Thresholds​

You can require confidence at the arm level or on individual elements/properties.

Arm-Level Threshold​

let verdict = match [10 ~> 0.9, 20 ~> 0.6] {
  [a, b] ~> 0.8 => "high",
  _ => "low"
}

Per-Element Thresholds​

let verdict = match [10 ~> 0.9, 20 ~> 0.6] {
  [a ~> 0.85, b ~> 0.5] => "ok",
  _ => "no"
}

Per-Property Thresholds​

let verdict = match {score: 90 ~> 0.7} {
  {score: s ~> 0.8} => "high",
  _ => "low"
}

Scoping​

Bindings inside a match arm are scoped to that arm only.