Unitful-like unit parsing macro

Author: MilesCranmer

src/uparse.jl

@@ -4,7 +4,8 @@ import ..constructorof
 import ..DEFAULT_QUANTITY_TYPE
 import ..DEFAULT_DIM_TYPE
 import ..DEFAULT_VALUE_TYPE
-import ..Units: UNIT_SYMBOLS
+import ..Units: UNIT_SYMBOLS, UNIT_VALUES
+import ..Constants: CONSTANT_SYMBOLS, CONSTANT_VALUES
 import ..Constants
 
 function _generate_units_import()
@@ -34,11 +35,11 @@ the quantity corresponding to the speed of light multiplied by Hertz,
 squared.
 """
 function uparse(s::AbstractString)
-    return as_quantity(eval(Meta.parse(s)))::DEFAULT_QUANTITY_TYPE
+    return as_quantity(eval(map_to_scope(Meta.parse(s))))::DEFAULT_QUANTITY_TYPE
 end
 
 as_quantity(q::DEFAULT_QUANTITY_TYPE) = q
-as_quantity(x::Number) = convert(DEFAULT_QUANTITY_TYPE,  x)
+as_quantity(x::Number) = convert(DEFAULT_QUANTITY_TYPE, x)
 as_quantity(x) = error("Unexpected type evaluated: $(typeof(x))")
 
 """
@@ -54,7 +55,50 @@ the quantity corresponding to the speed of light multiplied by Hertz,
 squared.
 """
 macro u_str(s)
-    return esc(Meta.parse(s))
+    ex = Meta.parse(s)
+    return esc(map_to_scope(ex))
+end
+
+function map_to_scope(ex::Expr)
+    if ex.head == :call
+        ex.args[2:end] = map(map_to_scope, ex.args[2:end])
+        return ex
+    elseif ex.head == :tuple
+        ex.args[:] = map(map_to_scope, ex.args)
+        return ex
+    elseif ex.head == :.
+        if ex.args[1] == :Constants
+            @assert ex.args[2] isa QuoteNode
+            return lookup_constant(ex.args[2].value)
+        else
+            return ex
+        end
+    else
+        throw(ArgumentError("Unexpected expression: $ex. Only `:call`, `:tuple`, and `:.` are expected."))
+        return ex
+    end
+end
+function map_to_scope(sym::Symbol)
+    if sym in UNIT_SYMBOLS
+        return lookup_unit(sym)
+    elseif sym in CONSTANT_SYMBOLS
+        throw(ArgumentError("Found the symbol $sym. To access constants in a unit expression, access the `Constants` module. For example, `u\"Constants.h\"`."))
+        return sym
+    else
+        throw(ArgumentError("Symbol $sym not found in `Units` or `Constants`."))
+        return sym
+    end
+end
+function map_to_scope(ex)
+    return ex
+end
+function lookup_unit(ex::Symbol)
+    i = findfirst(==(ex), UNIT_SYMBOLS)::Int
+    return UNIT_VALUES[i]
+end
+function lookup_constant(ex::Symbol)
+    i = findfirst(==(ex), CONSTANT_SYMBOLS)::Int
+    return CONSTANT_VALUES[i]
 end
 
 end