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 value error("missing") appears for that field, unless the field is nullable (i.e., is a union type including type null), in which case the null value appears.

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 errors are encountered, then some or all of the resulting value will be embedded with structured errors and the result does not have the target type.

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:error("missing")}
{x:1,y:null::(string|null)}