Context

[barucden]

This PR introduces Context, a structure that holds configuration of the decimal arithmetic.

Eventually, the global variable DIGITS should be completely removed in favor of this newly-added structure.

Requires #79

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.60%. Comparing base (f3deb0b) to head (a9e51b0).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
+ Coverage   98.13%   99.60%   +1.47%     
==========================================
  Files          10       11       +1     
  Lines         214      255      +41     
==========================================
+ Hits          210      254      +44     
+ Misses          4        1       -3     

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[barucden]

@ViralBShah I wonder if it would be a better interface to have CONTEXT be a ScopedValue. The users would then do

@with CONTEXT => Context(precision=123, rounding=RoundUp) begin
   # user code
end

instead of

setprecision(Decimal, 123) do
    setrounding(Decimal, RoundUp) do
        # user code
    end
end

I think the ScopedValue approach is better. It even avoids things like

# In package A
module A
    using Decimals
    function f()
        setprecision(Decimal, 123)
    end
end

# In package B
module B
    using A
    function g()
        setprecision(Decimal, 456)
        f()
        # precision is 123, not 456
    end
end

One "downside" is that users might be used to setprecision/setrounding from Base.

[ViralBShah]

This sounds like a better idea than global state for sure! I think doing things differently from Base that is a better way , should be encouraged. And that often paves the way for Base to do new things later. @oscardssmith Any thoughts, or anyone else we can ask.

I don't think there is much of a design decision here, since this is natural - but sometimes it is good to bring these up on discourse.

[oscardssmith]

yeah this seems like a reasonable design. I think this is what BigFloat is moving to as well, right?

[barucden]

I am not sure about BigFloat but there is a very recent PR that turns global variables into ScopedValues: JuliaLang/julia#56378

[barucden]

ScopedValues require Julia 1.8 at minimum. Is it okay to lift our requirement from v1.6 to v1.8?

[oscardssmith]

yeah 1.10 is new LTS, so no one would complain if you lifted to 1.8

[barucden]

What do you think about the with_context function/macro? It's just for convenience

Macro:

using Decimals
Decimals.@with_context (Emax=384, Emin=-383, precision=9, rounding=RoundNearestTiesAway) begin
    @test -(dec"56267e-0") == dec"-56267"
end

instead of

using Decimals
using ScopedValues
@with Decimals.CONTEXT => Context(Emax=384, Emin=-383, precision=9, rounding=RoundNearestTiesAway) begin
    @test -(dec"56267e-0") == dec"-56267"
end

Function:

using Decimals
Decimals.with_context(f, Emax=384, Emin=-383, precision=9, rounding=RoundNearestTiesAway)

instead of

using Decimals
using ScopedValues
with(f, Decimals.CONTEXT => Decimals.Context(Emax=384, Emin=-383, precision=9, rounding=RoundNearestTiesAway))