# Notations and Precedence

The most basic syntax extension commands allow introducing new (or
overloading existing) prefix, infix, and postfix operators.

```lean
infixl:65   " + " => HAdd.hAdd  -- left-associative
infix:50    " = " => Eq         -- non-associative
infixr:80   " ^ " => HPow.hPow  -- right-associative
prefix:100  "-"   => Neg.neg
# set_option quotPrecheck false
postfix:max "⁻¹"  => Inv.inv
```

After the initial command name describing the operator kind (its
"fixity"), we give the *parsing precedence* of the operator preceded
by a colon `:`, then a new or existing token surrounded by double
quotes (the whitespace is used for pretty printing), then the function
this operator should be translated to after the arrow `=>`.

The precedence is a natural number describing how "tightly" an
operator binds to its arguments, encoding the order of operations. We
can make this more precise by looking at the commands above unfold to:

```lean
notation:65 lhs:65 " + " rhs:66 => HAdd.hAdd lhs rhs
notation:50 lhs:51 " = " rhs:51 => Eq lhs rhs
notation:80 lhs:81 " ^ " rhs:80 => HPow.hPow lhs rhs
notation:100 "-" arg:100 => Neg.neg arg
# set_option quotPrecheck false
notation:1024 arg:1024 "⁻¹" => Inv.inv arg  -- `max` is a shorthand for precedence 1024
```

It turns out that all commands from the first code block are in fact
command *macros* translating to the more general `notation` command.
We will learn about writing such macros below. Instead of a single
token, the `notation` command accepts a mixed sequence of tokens and
named term placeholders with precedences, which can be referenced on
the right-hand side of `=>` and will be replaced by the respective
term parsed at that position. A placeholder with precedence `p`
accepts only notations with precedence at least `p` in that place.
Thus the string `a + b + c` cannot be parsed as the equivalent of `a +
(b + c)` because the right-hand side operand of an `infixl` notation
has precedence one greater than the notation itself. In contrast,
`infixr` reuses the notation's precedence for the right-hand side
operand, so `a ^ b ^ c` *can* be parsed as `a ^ (b ^ c)`. Note that if
we used `notation` directly to introduce an infix notation like
```lean
# set_option quotPrecheck false
notation:65 lhs:65 " ~ " rhs:65 => wobble lhs rhs
```
where the precedences do not sufficiently determine associativity,
Lean's parser will default to right associativity. More precisely,
Lean's parser follows a local *longest parse* rule in the presence of
ambiguous grammars: when parsing the right-hand side of `a ~` in `a ~
b ~ c`, it will continue parsing as long as possible (as the current
precedence allows), not stopping after `b` but parsing `~ c` as well.
Thus the term is equivalent to `a ~ (b ~ c)`.

As mentioned above, the `notation` command allows us to define
arbitrary *mixfix* syntax freely mixing tokens and placeholders.
```lean
# set_option quotPrecheck false
notation:max "(" e ")" => e
notation:10 Γ " ⊢ " e " : " τ => Typing Γ e τ
```
Placeholders without precedence default to `0`, i.e. they accept
notations of any precedence in their place. If two notations overlap,
we again apply the longest parse rule:
```lean
notation:65 a " + " b:66 " + " c:66 => a + b - c
#eval 1 + 2 + 3  -- 0
```
The new notation is preferred to the binary notation since the latter,
before chaining, would stop parsing after `1 + 2`. If there are
multiple notations accepting the same longest parse, the choice will
be delayed until elaboration, which will fail unless exactly one
overload is type correct.
