E06070: type inference is not supported within generic/template type/functions

What went wrong

Type inference using '<-' cannot be used inside generic/template definitions. Generic code must use explicit type declarations because the compiler needs to know how types relate to the generic parameters.

Worked examples

Compilable examples demonstrating this error and the correct fix:

• ek9 -q 854 (Why can't I use type inference inside a generic class bod...)

How to fix it

1. Replace the inferred declaration 'name <- value' with an explicit type: 'name as T: value'
2. Use the generic type parameter (T, K, V, etc.) in declarations
3. Be explicit about all variable types inside generic code

Rationale

Inside generics, the compiler must track how every type relates to the generic parameters. Inference could hide type relationships that matter during parameterization (e.g., when T becomes String).

Explicit declaration patterns

value as T: expression       // T is the generic type parameter
list as List of T: List()    // Generic using generic type param
maybe as Optional of T: Optional()   // completed from the declared type

Common causes

• Using inference patterns from non-generic code
• Forgetting that generic context has stricter rules
• Assuming compiler can infer relationship to T

See also

• E06070: inference in generic context (this error)
• E04050: general type inference limitations

Examples

Every example is compiled by the EK9 compiler: the first fails exactly as captioned, the others fix it.

Fails with E06070: type inference is not supported within generic/template type/functions

#!ek9
defines module error.e06070.bad

  defines class

    Box of type T
      item as T: T()

      default Box()

      Box()
        -> initial as T
        item: initial

      //Inside a generic type, every declaration states its type

      check()
        -> candidate as T
        <- rtn <- candidate?

      count()
        <- rtn as Integer?
        start <- 1
        rtn: start

      default operator ?

Fixed: Declare the type explicitly inside a generic type or function

#!ek9
defines module error.e06070.good

  defines class

    Box of type T
      item as T: T()

      default Box()

      Box()
        -> initial as T
        item: initial

      check()
        -> candidate as T
        <- rtn as Boolean: candidate?

      count()
        <- rtn as Integer?
        start as Integer: 1
        rtn: start

      default operator ?

Questions and answers

These Q&A entries show E06070 and its fix:

• Q854: Why can't I use type inference inside a generic class body in EK9?