Apply a minimum price only if the product price hasn't been set.

← Control Flow · Ref: Q1223

The :=? assign-if-unset operator assigns a value ONLY if the target variable is currently unset. If the variable already has a value, the assignment is skipped.

  price <- lookupCatalogPrice()   //no catalog price: unset
  minimumPrice <- 9.99#USD
  price :=? minimumPrice
  //price is now 9.99 USD because it was unset

This is an ASSIGNMENT operator — it sets a variable. Use :=? where the value MAY be unset - here it comes from a lookup that can find nothing. Straight after 'price <- Money()' the compiler can see it is unset, so ':=?' would always apply: write ':=', or declare it with the value (E08097).

See Q1038 for :=? with config fallback chains.

Example

#!ek9
defines module qa.flow.guardassign.money

  defines function

    lookupCatalogPrice()
      <- rtn <- Money()
      //Simulates: no catalog price found

    lookupSuggestedPrice()
      <- rtn <- 24.99#USD

  defines program

    GuardAssignMoneyDemo()
      stdout <- Stdout()

      // === PRICE IS UNSET — :=? assigns the minimum ===

      //No catalog price: unset
      price <- lookupCatalogPrice()
      minimumPrice <- 9.99#USD
      price :=? minimumPrice
      stdout.println(`Product price: ${price}`)

      // === PRICE IS ALREADY SET — :=? is skipped ===

      existingPrice <- 49.99#USD
      existingPrice :=? minimumPrice
      stdout.println(`Existing price preserved: ${existingPrice}`)

      // === FALLBACK CHAIN — first set value wins ===

      listPrice <- lookupCatalogPrice()
      listPrice :=? lookupSuggestedPrice()
      listPrice :=? minimumPrice
      stdout.println(`List price: ${listPrice}`)

Common mistakes

E01073 — EK9 has no null. Use :=? to conditionally assign — it only sets the value when the variable is currently unset. See ek9 -h E01073 for details.

Incorrect:

      if price == null
        price := minimumPrice

Correct:

      price :=? minimumPrice

Other ways to ask this

How do I conditionally assign a Money value only when the variable is unset?
A product listing has an optional price — apply a default minimum if not set.
In Java I'd check if price == null before assigning a BigDecimal default. What does EK9 use?
Migrating from Python where I use 'price = price or Decimal(default)' — what is the EK9 pattern?

Coming from another language?

Java: if (price == null) price = new BigDecimal("9.99"). Python: price = price or Decimal('9.99') (fails for Decimal(0)). Kotlin: price = price ?: minimumPrice. Go: no built-in money type. EK9: price :=? minimumPrice — one operator, correct tri-state semantics.

Keywords: :=?, assignment, default, guarded, minimum, money, price, product