CODEWITHSUNDEEP
crystal-lang
Alex Craft
Alex Craft

Reputation: 15336

Why NamedTuple of same types are not compatible?

Why NamedTuple of the same type are not compatible?

alias Data = NamedTuple(
  title: String?
)

data : Data = { title: nil }     # Explicitly specifying that 
                                 # it has ` : Data` type 

proc = ->(data : Data){ p data } 
proc.call data                   # And yet it says it's not 
                                 # of `Data` type

Error

Error: type must be NamedTuple(title: String | Nil), not NamedTuple(title: Nil)

Upvotes: 1

Views: 90

Answers (2)

asterite
asterite

Reputation: 2926

This is a bug, please report it in the issue tracker. Thank you!

Upvotes: 2

Johannes Müller
Johannes Müller

Reputation: 5661

Your assumption is incorrect: data : Data restricts the type of the local variable to Data (which is an alias for NamedTuple(title: String | Nil)). This type restriction does not affect the value assignment on the right hand side. The literal value {type: nil} is typed as NamedTuple(title: Nil). This type is compatible with the type restriction to Data, so it can be assigned to the variable.

This implicitly covariant type restriction does not apply with proc types. I'm not sure about the reasoning behind that. It seems that this should be able to work. So maybe it just needs to be implemented.

You can however explicitly cast NamedTuple(Nil) to NamedTuple(String | Nil):

alias Data = NamedTuple(
  title: String?
)

data = { title: nil }.as(Data) # Cast named tuple to Data

proc = ->(data : Data){ p data } 
proc.call data

However, it is usually not a good idea to work with named tuples as regular data types. They are only recommended for very specific use cases such as named arguments. Defining custom structs (for example using the record macro) is a better idea and gives you a more powerful and flexible data type. I'd implement your example like this:

record Data,
  title : String?

data = Data.new(title: nil)

proc = ->(data : Data){ p data } 
proc.call data

Upvotes: 2

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