Special Types

static[T]

As their name suggests, static parameters must be constant expressions:

proc precompiledRegex(pattern: static string): RegEx =
  var res {.global.} = re(pattern)
  return res

precompiledRegex("/d+") # Replaces the call with a precompiled
                        # regex, stored in a global variable

precompiledRegex(paramStr(1)) # Error, command-line options
                              # are not constant expressions

For the purposes of code generation, all static params are treated as generic params - the proc will be compiled separately for each unique supplied value (or combination of values).

Static params can also appear in the signatures of generic types:

type
  Matrix[M,N: static int; T: Number] = array[0..(M*N - 1), T]
    # Note how `Number` is just a type constraint here, while
    # `static int` requires us to supply an int value

  AffineTransform2D[T] = Matrix[3, 3, T]
  AffineTransform3D[T] = Matrix[4, 4, T]

var m1: AffineTransform3D[float]  # OK
var m2: AffineTransform2D[string] # Error, `string` is not a `Number`

Please note that static T is just a syntactic convenience for the underlying generic type static[T]. The type param can be omitted to obtain the type class of all constant expressions. A more specific type class can be created by instantiating static with another type class.

One can force an expression to be evaluated at compile time as a constant expression by coercing it to a corresponding static type:

import std/math
echo static(fac(5)), " ", static[bool](16.isPowerOfTwo)

The compiler will report any failure to evaluate the expression or a possible type mismatch error.

typedesc[T]

In many contexts, Nim treats the names of types as regular values. These values exist only during the compilation phase, but since all values must have a type, typedesc is considered their special type.

typedesc acts as a generic type. For instance, the type of the symbol int is typedesc[int]. Just like with regular generic types, when the generic param is omitted, typedesc denotes the type class of all types. As a syntactic convenience, one can also use typedesc as a modifier.

Procs featuring typedesc params are considered implicitly generic. They will be instantiated for each unique combination of supplied types, and within the body of the proc, the name of each param will refer to the bound concrete type:

proc new(T: typedesc): ref T =
  echo "allocating ", T.name
  new(result)

var n = Node.new
var tree = new(BinaryTree[int])

When multiple type params are present, they will bind freely to different types. To force a bind-once behavior, one can use an explicit generic param:

proc acceptOnlyTypePairs[T, U](A, B: typedesc[T]; C, D: typedesc[U])

Once bound, type params can appear in the rest of the proc signature:

template declareVariableWithType(T: typedesc, value: T) =
  var x: T = value

declareVariableWithType int, 42

Overload resolution can be further influenced by constraining the set of types that will match the type param. This works in practice by attaching attributes to types via templates. The constraint can be a concrete type or a type class.

template maxval(T: typedesc[int]): int = high(int)
template maxval(T: typedesc[float]): float = Inf

var i = int.maxval
var f = float.maxval
when false:
  var s = string.maxval # error, maxval is not implemented for string

template isNumber(t: typedesc[object]): string = "Don't think so."
template isNumber(t: typedesc[SomeInteger]): string = "Yes!"
template isNumber(t: typedesc[SomeFloat]): string = "Maybe, could be NaN."

echo "is int a number? ", isNumber(int)
echo "is float a number? ", isNumber(float)
echo "is RootObj a number? ", isNumber(RootObj)

Passing typedesc is almost identical, just with the difference that the macro is not instantiated generically. The type expression is simply passed as a NimNode to the macro, like everything else.

import std/macros

macro forwardType(arg: typedesc): typedesc =
  # `arg` is of type `NimNode`
  let tmp: NimNode = arg
  result = tmp

var tmp: forwardType(int)

typeof operator

Note: typeof(x) can for historical reasons also be written as type(x) but type(x) is discouraged.

One can obtain the type of a given expression by constructing a typeof value from it (in many other languages this is known as the typeof operator):

var x = 0
var y: typeof(x) # y has type int

If typeof is used to determine the result type of a proc/iterator/converter call c(X) (where X stands for a possibly empty list of arguments), the interpretation, where c is an iterator, is preferred over the other interpretations, but this behavior can be changed by passing typeOfProc as the second argument to `typeof`:

iterator split(s: string): string = discard
proc split(s: string): seq[string] = discard

# since an iterator is the preferred interpretation, `y` has the type `string`:
assert typeof("a b c".split) is string

assert typeof("a b c".split, typeOfProc) is seq[string]