@check convenience syntax for record/field-level value constraints

[jrevels]

Split out from #20. With Legolas v0.5 (ref #54), we have a much nicer disciplined stance on validation-by-construction, so the path forward for convenient record-level check constraints is much simpler.

We should define a simple helper @check that can be used in the following manner:

@version FooV1 begin
    x::String # unchecked
    @check(y::Int, y > 0)
    @check(z::Any, a_predicate(z) && another_predicate(z, x))
    @check(k::Int = clamp(k, 1, 10), 
           some_other_predicate(k))
    @check begin
        # record-level predicate(s)
    end
end

where the field-level @check expansion goes from statements like

@version FooV1 begin
    ⋮
    @check(f::T = $rhs, $predicate)
    ⋮
end

...to statements like:

f::T = $rhs
$predicate || throw(CheckConstraintError(:FooV1, :f, f, $stringified_predicate))

...where CheckConstraintError might look like:

struct CheckConstraintError <: Exception
    record_name::Symbol
    field_name::Symbol
    predicate::String
    received::Any
end

function Base.showerror(io::IO, e::CheckConstraintError)
    print(io, """
              Value received for $(e.field_name) of $(e.record_name): $(e.received)
              Violates `@check` constraint: $(e.predicate)
              """)
end

[ericphanson]

Could this be done as a piece of syntax that @version understands, instead of as a new macro? To avoid clashes with ArgCheck.jl or other macros

[jrevels]

Yup, that’s the intention, updated the title/OP to clarify