Convertible relation

A type a is implicitly convertible to type b iff the following algorithm returns true:

proc isImplicitlyConvertible(a, b: PType): bool =
  if isSubtype(a, b) or isCovariant(a, b):
    return true
  if isIntLiteral(a):
    return b in {int8, int16, int32, int64, int, uint, uint8, uint16,
                 uint32, uint64, float32, float64}
  case a.kind
  of int:     result = b in {int32, int64}
  of int8:    result = b in {int16, int32, int64, int}
  of int16:   result = b in {int32, int64, int}
  of int32:   result = b in {int64, int}
  of uint:    result = b in {uint32, uint64}
  of uint8:   result = b in {uint16, uint32, uint64}
  of uint16:  result = b in {uint32, uint64}
  of uint32:  result = b in {uint64}
  of float32: result = b in {float64}
  of float64: result = b in {float32}
  of seq:
    result = b == openArray and typeEquals(a.baseType, b.baseType)
  of array:
    result = b == openArray and typeEquals(a.baseType, b.baseType)
    if a.baseType == char and a.indexType.rangeA == 0:
      result = b == cstring
  of cstring, ptr:
    result = b == pointer
  of string:
    result = b == cstring

Implicit conversions are also performed for Nim’s range type constructor.

Let a0, b0 of type T.

Let A = range[a0..b0] be the argument’s type, F the formal parameter’s type. Then an implicit conversion from A to F exists if a0 >= low(F) and b0 <= high(F) and both T and F are signed integers or if both are unsigned integers.

A type a is explicitly convertible to type b iff the following algorithm returns true:

proc isIntegralType(t: PType): bool =
  result = isOrdinal(t) or t.kind in {float, float32, float64}
proc isExplicitlyConvertible(a, b: PType): bool =
  result = false
  if isImplicitlyConvertible(a, b): return true
  if typeEqualsOrDistinct(a, b): return true
  if isIntegralType(a) and isIntegralType(b): return true
  if isSubtype(a, b) or isSubtype(b, a): return true

The convertible relation can be relaxed by a user-defined type converter.

converter toInt(x: char): int = result = ord(x)
var
  x: int
  chr: char = 'a'
# implicit conversion magic happens here
x = chr
echo x # => 97
# one can use the explicit form too
x = chr.toInt
echo x # => 97

The type conversion T(a) is an L-value if a is an L-value and typeEqualsOrDistinct(T, typeof(a)) holds.