# std/typetraits

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This module defines compile-time reflection procs for working with types.

Unstable API.

Example:

    import std/typetraits
    type A = enum a0 = 2, a1 = 4, a2
    type B = enum b0 = 2, b1, b2
    assert A is enum
    assert A is HoleyEnum
    assert A isnot OrdinalEnum
    assert B isnot HoleyEnum
    assert B is OrdinalEnum
    assert int isnot HoleyEnum
    type C[T] = enum h0 = 2, h1 = 4
    assert C[float] is HoleyEnum

# Imports

since, macros

# Types

    HoleyEnum = (not Ordinal) and enum

  Enum with holes. Source Edit

    OrdinalEnum = Ordinal and enum

  Enum without holes. Source Edit

    StaticParam[value] = object

  Used to wrap a static value in genericParams. Source Edit

# Procs

    proc arity(t: typedesc): int {.magic: "TypeTrait", ...raises: [], tags: [],
                                   forbids: [].}

  Returns the arity of `t`. This is the number of "type" components or the number of generic parameters a given type `t` has.

  Example:

      doAssert arity(int) == 0
      doAssert arity(seq[string]) == 1
      doAssert arity(array[3, int]) == 2
      doAssert arity((int, int, float, string)) == 4

  Source Edit

    proc distinctBase(T: typedesc; recursive: static bool = true): typedesc {.
        magic: "TypeTrait", ...raises: [], tags: [], forbids: [].}

  Returns the base type for distinct types, or the type itself otherwise. If `recursive` is false, only the immediate distinct base will be returned.

  See also:

  - distinctBase template

  Example:

      type MyInt = distinct int
      type MyOtherInt = distinct MyInt
      doAssert distinctBase(MyInt) is int
      doAssert distinctBase(MyOtherInt) is int
      doAssert distinctBase(MyOtherInt, false) is MyInt
      doAssert distinctBase(int) is int

  Source Edit

    proc genericHead(t: typedesc): typedesc {.magic: "TypeTrait", ...raises: [],
        tags: [], forbids: [].}

  Accepts an instantiated generic type and returns its uninstantiated form. A compile-time error will be produced if the supplied type is not generic.

  See also:

  - stripGenericParams proc

  Example:

      type
        Foo[T] = object
        FooInst = Foo[int]
        Foo2 = genericHead(FooInst)

      doAssert Foo2 is Foo and Foo is Foo2
      doAssert genericHead(Foo[seq[string]]) is Foo
      doAssert not compiles(genericHead(int))

      type Generic = concept f
        type _ = genericHead(typeof(f))

      proc bar(a: Generic): typeof(a) = a

      doAssert bar(Foo[string].default) == Foo[string]()
      doAssert not compiles bar(string.default)

      when false: # these don't work yet
        doAssert genericHead(Foo[int])[float] is Foo[float]
        doAssert seq[int].genericHead is seq

  Source Edit

    proc hasClosure(fn: NimNode): bool {....raises: [], tags: [], forbids: [].}

  Returns true if the func/proc/etc `fn` has `closure`. `fn` has to be a resolved symbol of kind `nnkSym`. This implies that the macro that calls this proc should accept `typed` arguments and not `untyped` arguments. Source Edit

    proc hasDefaultValue(t: typedesc): bool {.magic: "TypeTrait", ...raises: [],
        tags: [], forbids: [].}

  Returns true if `t` has a valid default value.

  Example:

      {.experimental: "strictNotNil".}
      type
        NilableObject = ref object
          a: int
        Object = NilableObject not nil
        RequiresInit[T] = object
          a {.requiresInit.}: T

      assert hasDefaultValue(NilableObject)
      assert not hasDefaultValue(Object)
      assert hasDefaultValue(string)
      assert not hasDefaultValue(var string)
      assert not hasDefaultValue(RequiresInit[int])

  Source Edit

    proc isNamedTuple(T: typedesc): bool {.magic: "TypeTrait", ...raises: [], tags: [],
                                           forbids: [].}

  Returns true for named tuples, false for any other type.

  Example:

      doAssert not isNamedTuple(int)
      doAssert not isNamedTuple((string, int))
      doAssert isNamedTuple(tuple[name: string, age: int])

  Source Edit

    proc name(t: typedesc): string {.magic: "TypeTrait", ...raises: [], tags: [],
                                     forbids: [].}

  Returns the name of `t`.

  Alias for system.`$`(t) since Nim v0.20.

  Example:

      doAssert name(int) == "int"
      doAssert name(seq[string]) == "seq[string]"

  Source Edit

    proc rangeBase(T: typedesc[range]): typedesc {.magic: "TypeTrait", ...raises: [],
        tags: [], forbids: [].}

  Returns the base type for range types, or the type itself otherwise.

  See also:

  - rangeBase template

  Example:

      type MyRange = range[0..5]
      type MyEnum = enum a, b, c
      type MyEnumRange = range[b..c]
      doAssert rangeBase(MyRange) is int
      doAssert rangeBase(MyEnumRange) is MyEnum
      doAssert rangeBase(range['a'..'z']) is char

  Source Edit

    proc stripGenericParams(t: typedesc): typedesc {.magic: "TypeTrait", ...raises: [],
        tags: [], forbids: [].}

  This trait is similar to genericHead, but instead of producing an error for non-generic types, it will just return them unmodified.

  Example:

      type Foo[T] = object

      doAssert stripGenericParams(Foo[string]) is Foo
      doAssert stripGenericParams(int) is int

  Source Edit

    proc supportsCopyMem(t: typedesc): bool {.magic: "TypeTrait", ...raises: [],
        tags: [], forbids: [].}

  Returns true if `t` is safe to use for copyMem.

  Other languages name a type like these blob.

  Source Edit

    proc tupleLen(T: typedesc[tuple]): int {.magic: "TypeTrait", ...raises: [],
        tags: [], forbids: [].}

  Returns the number of elements of the tuple type `T`.

  See also:

  - tupleLen template

  Example:

      doAssert tupleLen((int, int, float, string)) == 4
      doAssert tupleLen(tuple[name: string, age: int]) == 2

  Source Edit

# Macros

    macro enumLen(T: typedesc[enum]): int

  Returns the number of items in the enum `T`.

  Example:

      type Foo = enum
        fooItem1
        fooItem2

      doAssert Foo.enumLen == 2

  Source Edit

# Templates

    template distinctBase[T](a: T; recursive: static bool = true): untyped

  Overload of distinctBase for values.

  Example:

      type MyInt = distinct int
      type MyOtherInt = distinct MyInt
      doAssert 12.MyInt.distinctBase == 12
      doAssert 12.MyOtherInt.distinctBase == 12
      doAssert 12.MyOtherInt.distinctBase(false) is MyInt
      doAssert 12.distinctBase == 12

  Source Edit

    template elementType(a: untyped): typedesc

  Returns the element type of `a`, which can be any iterable (over which you can iterate).

  Example:

      iterator myiter(n: int): auto =
        for i in 0 ..< n:
          yield i

      doAssert elementType(@[1,2]) is int
      doAssert elementType("asdf") is char
      doAssert elementType(myiter(3)) is int

  Source Edit

    template genericParams(T: typedesc): untyped

  Returns the tuple of generic parameters for the generic type `T`.

  Note: For the builtin array type, the index generic parameter will always become a range type after it's bound to a variable.

  Example:

      type Foo[T1, T2] = object

      doAssert genericParams(Foo[float, string]) is (float, string)

      type Bar[N: static float, T] = object

      doAssert genericParams(Bar[1.0, string]) is (StaticParam[1.0], string)
      doAssert genericParams(Bar[1.0, string]).get(0).value == 1.0
      doAssert genericParams(seq[Bar[2.0, string]]).get(0) is Bar[2.0, string]
      var s: seq[Bar[3.0, string]]
      doAssert genericParams(typeof(s)) is (Bar[3.0, string],)

      doAssert genericParams(array[10, int]) is (StaticParam[10], int)
      var a: array[10, int]
      doAssert genericParams(typeof(a)) is (range[0..9], int)

  Source Edit

    template get(T: typedesc[tuple]; i: static int): untyped

  Returns the `i`-th element of `T`.

  Example:

      doAssert get((int, int, float, string), 2) is float

  Source Edit

    template pointerBase[T](_: typedesc[ptr T | ref T]): typedesc

  Returns `T` for `ref T | ptr T`.

  Example:

      assert (ref int).pointerBase is int
      type A = ptr seq[float]
      assert A.pointerBase is seq[float]
      assert (ref A).pointerBase is A # not seq[float]
      assert (var s = "abc"; s[0].addr).typeof.pointerBase is char

  Source Edit

    template rangeBase[T: range](a: T): untyped

  Overload of rangeBase for values.

  Example:

      type MyRange = range[0..5]
      type MyEnum = enum a, b, c
      type MyEnumRange = range[b..c]
      let x = MyRange(3)
      doAssert rangeBase(x) is int
      doAssert $typeof(rangeBase(x)) == "int"
      doAssert rangeBase(x) == 3
      let y: set[MyEnumRange] = {c}
      for e in y:
        doAssert rangeBase(e) is MyEnum
        doAssert $typeof(rangeBase(e)) == "MyEnum"
        doAssert rangeBase(e) == c
      let z: seq[range['a'..'z']] = @['c']
      doAssert rangeBase(z[0]) is char
      doAssert $typeof(rangeBase(z[0])) == "char"
      doAssert rangeBase(z[0]) == 'c'

  Source Edit

    template toSigned(T: typedesc[SomeInteger and not range]): untyped

  Returns a signed type with same bit size as `T`.

  Example:

      assert int8.toSigned is int8
      assert uint16.toSigned is int16
      # range types are currently unsupported:
      assert not compiles(toSigned(range[0..7]))

  Source Edit

    template toUnsigned(T: typedesc[SomeInteger and not range]): untyped

  Returns an unsigned type with same bit size as `T`.

  Example:

      assert int8.toUnsigned is uint8
      assert uint.toUnsigned is uint
      assert int.toUnsigned is uint
      # range types are currently unsupported:
      assert not compiles(toUnsigned(range[0..7]))

  Source Edit

    template tupleLen(t: tuple): int

  Returns the number of elements of the tuple `t`.

  See also:

  - tupleLen proc

  Example:

      doAssert tupleLen((1, 2)) == 2

  Source Edit

# Exports

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