Function Calls (Parameterized Fields)

When a field generator function accepts parameters, you must provide those arguments in a calls section. This enables you to create flexible, configurable field generators while keeping the generation logic clean and reusable.

Basic Structure

The pattern consists of three parts:

Field declaration with parameters
Generator function that accepts those parameters
Calls section that provides the arguments

model Session {
  // 1. Declare field with parameters
  fields {
    created_at(start time.Time, end time.Time) time.Time
  }

  // 2. Define generator function
  gens {
    func created_at(start time.Time, end time.Time) {
      return DateBetween(start, end)
    }
  }

  // 3. Provide arguments
  calls {
    created_at(time.Now().AddDate(-1, 0, 0), time.Now())
  }
}

Running the example:

datagenc gen Session.dg -n 3

Output:

Session{created_at:{wall:205915000 ext:63892967191 loc:0x10140eb40}}
Session{created_at:{wall:205915000 ext:63884181274 loc:0x10140eb40}}
Session{created_at:{wall:205915000 ext:63890140992 loc:0x10140eb40}}

How It Works

Argument Evaluation

Arguments in the calls section are evaluated once at the start and reused for all generated records.

calls {
  created_at(time.Now(), time.Now().Add(24*time.Hour))
  // These timestamps are calculated once, not per record
}

Type Matching

Argument types and counts must match the field signature exactly:

// Correct: Types match
fields {
  user_id(min int, max int) int
}
calls {
  user_id(1000, 2000)
}

// Incorrect: Type mismatch
calls {
  user_id("1000", "2000")  // Strings instead of ints
}

// Incorrect: Wrong number of arguments
calls {
  user_id(1000)  // Missing second argument
}

Common Pitfalls

Forgetting to Declare Parameters

// Incorrect: Parameters in gens but not in fields
fields {
  user_id() int  // Missing parameters
}

gens {
  func user_id(min int, max int) {
    return IntBetween(min, max)
  }
}

Missing calls Entry

// Incomplete: No calls entry for parameterized field
fields {
  user_id(min int, max int) int
}

gens {
  func user_id(min int, max int) {
    return IntBetween(min, max)
  }
}

// Missing: calls section!

Dynamic Evaluation Assumption

// Misunderstanding: time.Now() is evaluated once, not per record
calls {
  timestamp(time.Now(), time.Now().Add(24*time.Hour))
  // All records will use the same timestamp range
}

// Use iter for per-record variation instead
gens {
  func timestamp(baseTime time.Time, interval time.Duration) {
    return baseTime.Add(time.Duration(iter) * interval)
  }
}

calls {
  timestamp(time.Now(), time.Hour)
}