Simple affine units implementation

README.md

@@ -18,10 +18,11 @@ This can greatly improve both runtime performance, by avoiding type instabilitie
 - [Usage](#usage)
   - [Constants](#constants)
   - [Symbolic Units](#symbolic-units)
+    - [Custom Units](#custom-units)
+    - [Affine Units](#affine-units)
   - [Arrays](#arrays)
   - [Unitful](#unitful)
 - [Types](#types)
-- [Vectors](#vectors)
 
 ## Performance
 
@@ -284,6 +285,18 @@ julia> 3us"V" |> us"OneFiveV"
 2.0 OneFiveV
 ```
 
+#### Affine Units
+
+You can also use "*affine*" units such as Celsius or Fahrenheit,
+using the `ua"..."` string macro:
+
+```julia
+julia> room_temp = 22ua"degC"
+295.15 K
+
+julia> freezing = 32ua"degF"
+273.15 K
+```
 
 ### Arrays
 

docs/src/units.md

@@ -41,3 +41,32 @@ You can define custom units with the `@register_unit` macro:
 ```@docs
 @register_unit
 ```
+
+## Affine Units
+
+DynamicQuantities also supports affine units like Celsius and Fahrenheit through the `AffineUnit{R}` type and the `ua` string macro.
+For example,
+
+```julia
+# Define temperature in Celsius
+room_temp = 22ua"degC"    # 295.15 K
+
+# Define temperature in Fahrenheit
+freezing = 32ua"degF"     # 273.15 K
+
+# Can take differences normally, as these are now regular Quantities:
+room_temp - freezing
+# 22 K
+```
+
+Note there are some subtleties about working with these:
+
+```@docs
+@ua_str
+aff_uparse
+```
+
+Currently, the only supported affine units are:
+
+- `°C` or `degC` - Degrees Celsius
+- `°F` or `degF` - Degrees Fahrenheit

src/DynamicQuantities.jl

@@ -10,7 +10,7 @@ export QuantityArray
 export DimensionError
 export ustrip, dimension, uexpand, uconvert, ustripexpand
 export ulength, umass, utime, ucurrent, utemperature, uluminosity, uamount
-export uparse, @u_str, sym_uparse, @us_str, @register_unit
+export uparse, @u_str, sym_uparse, @us_str, @register_unit, aff_uparse, @ua_str
 
 # Deprecated:
 export expand_units
@@ -29,6 +29,7 @@ using DispatchDoctor: @stable
     include("constants.jl")
     include("uparse.jl")
     include("symbolic_dimensions.jl")
+    include("affine_dimensions.jl")
     include("complex.jl")
     include("register_units.jl")
     include("disambiguities.jl")

src/affine_dimensions.jl

@@ -0,0 +1,116 @@
+"""
+    AffineUnit{R}
+
+A simple struct for representing affine units like Celsius and Fahrenheit.
+This is not part of the AbstractDimensions hierarchy.
+
+AffineUnit only supports scalar multiplication in the form `number * unit` (e.g., `22ua"degC"`),
+which immediately converts it to a regular `Quantity{Float64,Dimensions{R}}`. Other operations
+like `unit * number`, division, addition, or subtraction with AffineUnit are not supported.
+
+!!! warning "Non-associative multiplication"
+    Multiplication with AffineUnit is non-associative due to the auto-conversion property.
+    For example, `(2 * 3) * ua"degC"` ≠ `2 * (3 * ua"degC")` because when a number multiplies an AffineUnit,
+    it immediately converts to a regular Quantity with the affine transformation applied.
+
+!!! warning
+    This is an experimental feature and may change in the future.
+"""
+struct AffineUnit{R}
+    scale::Float64
+    offset::Float64
+    basedim::Dimensions{R}
+    name::Symbol
+end
+
+Base.show(io::IO, unit::AffineUnit) = print(io, unit.name)
+
+# This immediately converts to regular Dimensions
+function Base.:*(value::Number, unit::AffineUnit)
+    # Apply the affine transformation: value * scale + offset
+    new_value = value * unit.scale + unit.offset
+    # Always use Float64 for temperature conversions to avoid precision issues
+    return Quantity(new_value, unit.basedim)
+end
+
+# Error messages for unsupported operations - defined using a loop
+for op in [:*, :/, :+, :-], (first, second) in [(:AffineUnit, :Number), (:Number, :AffineUnit)]
+
+    # Skip the already defined value * unit case
+    op == :* && first == :Number && second == :AffineUnit && continue
+
+    @eval function Base.$op(a::$first, b::$second)
+        throw(ArgumentError("Affine units only support scalar multiplication in the form 'number * unit', e.g., 22 * ua\"degC\", which will immediately convert it to a regular `Quantity{Float64,Dimensions{R}}`. Other operations are not supported."))
+    end
+end
+
+# Module for affine unit parsing
+module AffineUnits
+    import ..AffineUnit
+    import ..Dimensions
+    import ..DEFAULT_DIM_BASE_TYPE
+    import ..Quantity
+
+    # Define Celsius and Fahrenheit units inside the module
+    const °C = AffineUnit(1.0, 273.15, Dimensions{DEFAULT_DIM_BASE_TYPE}(temperature=1), :°C)
+    const degC = °C
+    const °F = AffineUnit(5/9, 459.67 * 5/9, Dimensions{DEFAULT_DIM_BASE_TYPE}(temperature=1), :°F)
+    const degF = °F
+
+    const AFFINE_UNIT_SYMBOLS = [:°C, :degC, :°F, :degF]
+
+    function map_to_scope(ex::Expr)
+        if ex.head != :call
+            throw(ArgumentError("Unexpected expression: $ex. Only `:call` is expected."))
+        end
+        ex.args[2:end] = map(map_to_scope, ex.args[2:end])
+        return ex
+    end
+
+    function map_to_scope(sym::Symbol)
+        if !(sym in AFFINE_UNIT_SYMBOLS)
+            throw(ArgumentError("Symbol $sym not found in affine units. Only °C/degC and °F/degF are supported."))
+        end
+        if sym in (:°C, :degC)
+            return °C
+        else  # if sym in (:°F, :degF)
+            return °F
+        end
+    end
+
+    # For literals and other expressions
+    map_to_scope(ex) = ex
+end
+
+"""
+    ua"unit"
+
+Parse a string containing an affine unit expression.
+Currently only supports °C (or degC) and °F (or degF).
+
+For example:
+
+```julia
+room_temp = 22ua"degC"  # The multiplication returns a Quantity
+```
+
+!!! warning
+    This is an experimental feature and may change in the future.
+"""
+macro ua_str(s)
+    ex = AffineUnits.map_to_scope(Meta.parse(s))
+    return esc(ex)
+end
+
+"""
+    aff_uparse(s::AbstractString)
+
+Parse a string into an affine unit (°C/degC, °F/degF). Function equivalent of `ua"unit"`.
+
+!!! warning
+    This is an experimental feature and may change in the future.
+"""
+function aff_uparse(s::AbstractString)
+    ex = AffineUnits.map_to_scope(Meta.parse(s))
+    return eval(ex)::AffineUnit{DEFAULT_DIM_BASE_TYPE}
+end

test/unittests.jl

@@ -6,6 +6,7 @@ using DynamicQuantities: GenericQuantity, with_type_parameters, constructorof
 using DynamicQuantities: promote_quantity_on_quantity, promote_quantity_on_value
 using DynamicQuantities: UNIT_VALUES, UNIT_MAPPING, UNIT_SYMBOLS, ALL_MAPPING, ALL_SYMBOLS, ALL_VALUES
 using DynamicQuantities.SymbolicUnits: SYMBOLIC_UNIT_VALUES
+using DynamicQuantities: AffineUnit, AffineUnits
 using DynamicQuantities: map_dimensions
 using DynamicQuantities: _register_unit
 using Ratios: SimpleRatio
@@ -2000,6 +2001,61 @@ end
     @test QuantityArray([km, km]) |> uconvert(us"m") != [km, km]
 end
 
+
+@testset "Tests of AffineDimensions" begin
+    # Test basic unit creation
+    °C = ua"°C"
+    °F = ua"°F"
+
+    # Test unit identity
+    @test °C isa AffineUnit
+
+    # Test basic properties
+    @test °C.basedim.temperature == 1
+    @test °C.basedim.length == 0
+
+    # Test unit equivalence
+    @test ua"°C" == ua"degC"
+    @test ua"°F" == ua"degF"
+
+    # Test conversion to regular dimensions via multiplication
+    @test 0 * °C ≈ 273.15u"K"
+    @test 100 * °C ≈ 373.15u"K"
+    @test 32 * °F ≈ 273.15u"K"
+
+    # Test temperature equivalence
+    @test 0ua"degC" ≈ 32ua"degF"
+    @test -40ua"degC" ≈ -40ua"degF"
+
+    # Can do multiplication inside
+    @test ua"22degC" isa Quantity
+    @test ua"22degC" == 22ua"degC"
+
+    # Test unsupported operations - verify the error message
+    @test_throws "Affine units only support scalar multiplication in the form 'number * unit'" °C * 2
+
+    # Test AffineUnits module functionality
+    @test AffineUnits.°C === °C
+    @test AffineUnits.degC === °C
+    @test AffineUnits.°F === °F
+    @test AffineUnits.degF === °F
+
+    # Test parsing of non-:call expression
+    @test_throws "Unexpected expression" AffineUnits.map_to_scope(:(let x=1; x; end))
+
+    # Test aff_uparse function
+    @test aff_uparse("°C") === ua"°C"
+    @test aff_uparse("degC") === ua"degC"
+    @test aff_uparse("°F") === ua"°F"
+    @test aff_uparse("degF") === ua"degF"
+    @test_throws ArgumentError aff_uparse("K")
+
+    # Test show function for AffineUnit
+    @test sprint(show, °C) == "°C"
+
+    @test sprint(show, °F) == "°F"
+end
+
 @testset "Test div" begin
     for Q in (RealQuantity, Quantity, GenericQuantity)
         x = Q{Int}(10, length=1)