typetraits

nimony/lib/std/typetraits.nim

Compile-time reflection on types.

Nim 2 implemented these as {.magic: "TypeTrait".} procs, i.e. as compiler built-ins. Nimony implements them as ordinary template plugins: the compiler hands the plugin the argument type as NIF, the plugin transforms that tree, and the result is re-checked in type position. Nothing here is known to the compiler.

A plugin runs in its own process and so cannot resolve a symbol on its own. It does not have to: it asks. When the plugin meets a nominal type it has not been given, it returns needTypes(sym), the compiler appends that declaration to its second input and runs it again. Nothing is shipped that was not asked for.

Unstable API.

template distinctBase(t: typedesc[T]): typedesc
Returns the base type for distinct types, or the type itself otherwise. Peels every distinct layer; see distinctBaseShallow for a single step.
.. code-block:: nim type MyInt = distinct int type MyOtherInt = distinct MyInt var a: distinctBase(MyOtherInt) # int
template distinctBaseShallow(t: typedesc[T]): typedesc
Like distinctBase but removes only the immediate distinct layer. (Nim 2 spelled this distinctBase(T, recursive = false).)
.. code-block:: nim type MyInt = distinct int type MyOtherInt = distinct MyInt var a: distinctBaseShallow(MyOtherInt) # MyInt
template genericHead(t: typedesc[T]): typedesc
Accepts an instantiated generic type and returns its uninstantiated form. It is a compile-time error if T is not a generic instance.
.. code-block:: nim type Foo[T] = object var x: genericHead(Foo[int])[float] # Foo[float]
template stripGenericParams(t: typedesc[T]): typedesc
Like genericHead, but returns non-generic types unmodified instead of producing an error.