# Problems with Minus

It’s common to write negatives using a “minus sign”, e.g. the negative of 5 as  − 5 and the negative of x as  − x. Here are some reasons I prefer the “bar” notation.

## Minus Signs are Ambiguous

The minus sign has two common usages:

• A single value prefixed with a minus sign indicates a negative, e.g.  − 5 means the same as \$\ngtv{5}\$. (This is sometimes called “unary minus”, since it applies to a single value)
• A pair of values separated by a minus sign indicates subtraction, e.g. 5 − 2 means “five take away two”.

This is a reasonable distinction on its own, but causes problems when expressions involve multiple values and operations. Whilst these can be avoided using various conventions and workarounds, they make it harder to learn, harder to teach, harder to program into a computer (especially if we don’t want extra parentheses “just in case”!), etc.

For example:

• It’s common to write multiplication by putting values side-by-side, e.g. 2x to mean “2 times x”. Minus signs make this ambiguous, since 2 − x could mean “two times negative xor it could mean “two take away x”; these are two very different things! Convention is to always treat such minus signs as subtraction, and use parentheses if we want multiplication, e.g. 2( − x)

## Problems with Unary Minus

The usual way to indicate negatives is with a “minus sign” , but that notation can be confusing, and misses somenegatives by writing a lbar

things (where “positive” just means normal/standard/usual/everyday/etc.).

• If a situation is easily modelled using Nat then we should stick to that, and not introduce negative numbers, fractions, complex numbers, etc. since they’re not needed.
• Example: Alice has 2 apples, Bob gives Alice 3 apples, how many apples does Alice have?
• Alice can’t have ‘negative apples’, and Bob can only increase their apple count, so negatives aren’t needed (and likewise for fractional apples, imaginary apples, infinitesimal apples, transfinite cardinals of apples, etc.)
• Note that many programming languages get this wrong: e.g. having ‘length’ return a Z, forcing callers to account for the negative case.
• When a situation does make sense with negatives, we should use them.
• For example: if Alice has 3 apples and gives 2 to Bob, how many apples does Alice have left?