Boolean subset confusion


Forgive if this topic is covered extensively in a multitude of different locations, but perhaps this will get everyone thinking fundamental language implementation:

why is a Boolean a subset of Integer with 8 bits.

Clearly only one bit is needed, so what’s the purpose of dedicating 7 more bits to the interpreter


It is unclear what you mean by this. Subtype? But Bool isn’t a subtype of either Int8 or UInt8.

It is, however, implemented as an 8-bit primitive type:

The declaration of Bool above therefore means that a boolean value takes eight bits to store, and has Integer as its immediate supertype. Currently, only sizes that are multiples of 8 bits are supported. Therefore, boolean values, although they really need just a single bit, cannot be declared to be any smaller than eight bits.

What interpreter?

Also, see BitArray. The choice between that and an array of Bool depends on whether speed or memory use is more important, as querying and flipping a single bit is comparatively more expensive.


Computers typically work in units of Bytes and not in units of Bits, and 1 byte = 8 bits. Therefore, multiples of 8 bits or 1 byte is used.

You can’t address less than 8 bits in memory.



Thank you


I meant to write subtype not subset


@chakravala okay I think I get it, so although there are 256 possible values, 1 byte is the minimum size that can be addressed? So back to the Boolean
julia> x = true::Bool

julia> bitstring(x)

julia> y = false::Bool

julia> bitstring(y)


Perhaps you meant the compiler.

Did you expect a different interpretation? What do you want to use it for?

If you just want to use ::Bool values in computation, they behave like the number 0 and 1 for practical purposes, except that they have a specific type, which is required in some contexts (eg if), and cannot accept any other value (I am ignoring some corner cases here).

The actual representation is not that important, except if you are interfacing with C or in some corner cases.

(Also, please quote your code.)


@Tamas_Papp thanks for the input