GALA-E0067 — Type argument cannot be inferred

When it fires. A generic type is constructed and nothing at the call determines one of its type parameters: not the arguments, not the binding’s declared type, not the enclosing function’s result type. It covers a generic struct built from its fields (Tag("x")), a type called through its companion Apply (Try(x)), and a partial type-argument list (Mk[int](2, "c")) whose remaining parameters the arguments do not fix.

Minimal repro.

package main

struct Tag[T any](Name string)

func main() {
    val t = Tag("x")
    Println(t.Name)
}

Error output.

error[GALA-E0067]: cannot infer type argument T of generic struct Tag from its fields or the expected type; annotate the binding (e.g. `val x Tag[int] = Tag(...)`) or write it explicitly (`Tag[int](...)`)
  --> main.gala:6:17
  |
6 |     val t = Tag("x")
  |                 ^^^
  |

Fix. Say which type you mean, on the binding or on the call:

package main

struct Tag[T any](Name string)

func main() {
    val t Tag[int] = Tag("x")
    Println(t.Name)
}

An argument whose type is unknown. The same error fires when an argument should have fixed the type parameter but has no type itself. The usual cause is a call into a Go package whose type information GALA could not load, so the result type of the call is unknown:

package main

import "golang.org/x/term"

func main() {
    Println(Try(term.MakeRaw(0)).IsSuccess())
}

error[GALA-E0067]: cannot infer type argument T of Try: the type of its argument `term.MakeRaw(...)` is unknown
  --> main.gala:6:17
  |
6 |     Println(Try(term.MakeRaw(0)).IsSuccess())
  |                 ^^^^ the type information of Go package "golang.org/x/term" could…
  |
  = hint: the type information of Go package "golang.org/x/term" could not be loaded; require its module in gala.mod (`gala mod add --go <module>`)

Require the Go module in the gala.mod of the module whose code imports it:

require golang.org/x/term v0.25.0 // go

gala build and gala test download a required Go module before they read its types, for the project and for every GALA library it depends on, so this form of the error means the module is not required, or could not be downloaded (offline, or a private module without credentials); a failed download is printed as a warning above the error.

Rationale. Go cannot use a generic type without its type arguments. A companion Apply call used to be emitted as written — std.Try{}.Apply(term.MakeRaw(0)) — and the program failed in go build with cannot use generic type std.Try[T any] without instantiation, a message about generated Go rather than about the GALA line that caused it.

What still works. Any construction whose type arguments the arguments, the declared type of the binding or the enclosing function’s result type determine: Tag[int]("x"), val t Tag[int] = Tag("x"), and Try(term.MakeRaw(fd)) once golang.org/x/term is required.