The symbols of q!¶

q!(100 m/s) names its unit by symbol. Each symbol is one unit marker, and one that could mean two units doesn't compile: min is a minute and a US or imperial minim, so q! asks which one.

use exact_units::{Fixed, q, prefix::Milli, units::{Minute, Volt}};

let v = Fixed::<Volt, Milli, u16>::new(3300);
assert!(v > q!(3 V));
assert!(v == q!(3300 mV));
assert!(v < q!(0.004 kV));

let t = Fixed::<Minute, Milli, u32>::new(1500);
assert!(t == q!(1.5 Minute)); // `min` reads more than one way: the marker says which

use exact_units::q;

let _ = q!(5 min); // a minute or a minim?

Where the table comes from¶

The table is generated from the crate's unit! and origin! lines, so a new marker gets its symbol without a second list to keep. just q-symbols writes it (macros/src/pure/table.rs), the tables of this page and the names and quantities this book's unit tooltips show (docs_ext/eu_docs_ext/unit_info.json); just test fails when any is stale. The rules:

• One word. A unit! symbol is a q! symbol when it has no /, ·, space, parenthesis, digit or exponent. m/s² is written m/s^2, a product of symbols. A qualified symbol (gal (US)) is written as its marker, GallonUs.
• Prefixes where the SI allows them. A coherent SI unit takes the SI prefixes, and so do g, L and eV. The tonne takes them from kilo up (kt, Mt): a smaller mass is written in grams. bit and B take the binary ones too (KiB). Nothing else does: no kh, mdeg or kin.
• No bare default. A qualified symbol never lets its stem be read as one of its units: ft is refused while ft (US survey) exists, although the international foot's own symbol is ft, and qt is refused for the US liquid, US dry and imperial quarts. Words qualify like a parenthesis: liq and dry before a symbol, avdp, t and ap after one (oz t, the troy ounce), where the stem is no unit's own symbol (s ap, the scruple, leaves s the second). A stem with one qualified unit is refused too (cable): write the marker.
• Two readings, refused. A symbol's readings are its own lines, the markers of the qualified symbols it is the stem of, and every split into a prefix and a unit that takes that prefix. cd is the candela and the cord, so it is refused; it is no centi-day, as the day takes no prefix, and yd is the yard alone. mcd is a millicandela, never a millicord.
• The same value folds. Readings of the same dimension, kind and exact scale are one: km is the marker Kilometre and kilo Meter, so it is read as kilo Meter.
• Every group counts. The table is worked out with every feature on. Which symbols are refused never depends on the features a build turns on; a symbol whose group is off is an error that names the feature.

Text a person types at run time (Exact::from_str, serde::with_unit) reads each of these symbols as q! does or refuses it, never as another value; a test checks every one. Only q! reads the ASCII spellings ohm, deg and percent and the binary prefixes past Ti.

Units¶

The symbols q! reads as a unit marker, and the feature each needs. %, ‰, ppm and ppb are a Ratio with a power of ten.

Symbols that are a prefix and a unit¶

A unit! line whose symbol is also a prefix on another unit of the same value is read as the prefix and that unit.

Refused symbols¶

Each reads more than one way, so q! refuses it and names the readings. Write the marker you mean as a type: q!(5 Candela), q!(1 GallonUs).

Origins¶

A temperature in °C, °F or °R is a point against its origin, and compares with a Point only. Each origin has an ASCII spelling written bare, q!(25 degC); the others hold a character Rust's lexer doesn't take (°, ℃), so they are written in quotes, q!(25 "°C").

Levels¶

dB and Np alone after the number are a level of a gain, q!(-3 dB), and compare with a linear gain whose kind has a level form: a ratio of root-power quantities (V/V, A/A, Pa/Pa) reads \(20 \lg\), one of powers (W/W) \(10 \lg\), so the symbol reads one way there. The compare is exact: gain < q!(-3 dB) is one compare against the floor of \(10^{-3/20}\) in the gain's steps, worked out at compile time.

use exact_units::{Fixed, q, prefix::Milli, units::{Per, Volt, Watt}};

let v = Fixed::<Per<Volt, Volt>, Milli, u16>::new(707);
assert!(v < q!(-3 dB)); // 0.70794.. V/V
let p = Fixed::<Per<Watt, Watt>, Milli, u16>::new(501);
assert!(p < q!(-3 dB)); // 0.50118.. W/W
assert!(p > q!(-0.35 Np)); // e^(−0.7) = 0.49658.. W/W

Against a value stored as a level (Decibel, Loss<Decibel>, Neper) or inside a unit (dB/km) they stay refused, as a level of gain and one of loss differ in sign: write the marker, q!(-3 Decibel) or q!(3 Loss<Decibel>).

Written as a type¶

These unit! symbols are no q! symbol: a compound one is written with operators (V/m as V / m), a qualified one as its marker.