upcast

convert a value to a supertype

Synopsis

upcast(val: any, target: type) -> any

Description

The upcast function is like cast but does not perform any type coercion and converts a value val from its type to any supertype of its type as indicated by the target type argument.

When a record value does not contain a field in the super type, the super type’s corresponding field must be optional and the missing value will appear as “none”.

A type is a supertype of a subtype if all paths through the subtype are valid paths through the supertype.

Upcasting is used by the fuse operator.

When an upcast is successful, the return value of cast always has the target type.

If an error is encountered, the offending value and target type are returned in a structured error.

Examples

Upcast showing missing versus null

# spq
values
  upcast({x:1},<{x:int64,y?:string}>),
  upcast({x:1},<{x:int64,y?:string|null}>)
# input

# expected output
{x:1,y?:_::string}
{x:1,y?:_::(string|null)}