User defined default values

[pmetras]

User manual defines how variables are initialized. In fact, memory used by the variables is 0-filled.

Proposal

The special func default(t: typedesc[T]): T can be defined by the user to specify a different default value. When it exists, the compiler will call this proc to initialize variables of type T. In such case, 0-filled memory does not occur and it's replaced by default calls.

Justification

• Let the user select the default value. The following program won't display error message Warning: Cannot prove that 'f' is initialized. This will become a compile time error in the future. [ProveInit]

type
  Val = range['A' .. 'Z']

func default(a: typedesc[Val]): Val =
  'A'

proc foo =
  var f: Val
  echo "f = '", f, "', ord = ",  ord(f)

foo()

• Better control over default values for ranges, enum, tuples, arrays, etc.
• No change to existing behaviour and limited change to documentation.
• Some [ProveInit] warnings can be corrected by users.

[dom96]

I like this idea, since AFAIK the compiler already has a concept of default.

[simonkrauter]

Maybe related to:

• Support default values for object properties #126 Support default values for object properties
• [RFC] Another proposal for a standardized object construction/initialization. #48 Another proposal for a standardized object construction/initialization

[snej]

Great feature, but the syntax seems overly complex to me for something whose effect is so simple. How about proc =init(var T), since it’s functionally the opposite of the =destroy function? (insert your own backticks around the name; I can’t figure out how to escape them in Markdown.)

[iacore]

It should probably be =init.

On another though maybe using default(T) explicitly is better.