19.1 Elaboration of infix expressions

The following infix declamations give identifiers in the $\,\langle$ vid $\rangle\,$ class infix operator property.

infix $($ n $)?$ $\,\langle$ vidSeq $\rangle\,$
infixr $($ n $)?$ $\,\langle$ vidSeq $\rangle\,$

infix defines $\,\langle$ vidSeq $\rangle\,$ as left associative infix operators and infixr defines $\,\langle$ vidSeq $\rangle\,$ as right associative infix operators. Optional integer $($ n $)?$ (from $0$ to $9$ ) specifies association strength (with $9$ the strongest). If $n$ is omitted then $0$ is assumed. Declaration

nonfix $\,\langle$ vidSeq $\rangle\,$

cancel infix operator property of identifiers $\,\langle$ vidSeq $\rangle\,$ .

Infix expressions are converted to applications to tuples according to the association strength.

source	result
$\,\langle$ exp ${}_{1}$ $\rangle\,$ $\,\langle$ vid $\rangle\,$ $\,\langle$ exp ${}_{2}$ $\rangle\,$	op $\,\langle$ vid $\rangle\,$ ( $\,\langle$ exp ${}_{1}$ $\rangle\,$ , $\,\langle$ exp ${}_{2}$ $\rangle\,$ )
$\,\langle$ pat ${}_{1}$ $\rangle\,$ $\,\langle$ vid $\rangle\,$ $\,\langle$ pat ${}_{2}$ $\rangle\,$	op $\,\langle$ vid $\rangle\,$ ( $\,\langle$ pat ${}_{1}$ $\rangle\,$ , $\,\langle$ pat ${}_{2}$ $\rangle\,$ )

The syntax, when appears in expressions and patters,

op $\,\langle$ vid $\rangle\,$

cancel the infix status of $\,\langle$ vid $\rangle\,$ . Therefore if the identifier foo has infix status, then the following two code fragments are equivalent.

1 foo 2
op foo (1,2)

The following are implicitly declared in all the compilation unit and the interactive mode.

infix 7 * / div mod
infix 6 + - ^
infixr 5 :: @
infix 4 = <> > >= < <=
infix 3 := o
infix 0 before

The above hierarchical syntax with infix declarations determines the association strength of expressions. For example, record update expression ( $\,\langle$ exp ${}_{1}$ $\rangle\,$ # { $\,\langle$ lab $\rangle\,$ = $\,\langle$ exp ${}_{2}$ $\rangle\,$ }) associates tightly than strongest infix operators (those that are declared with infix 9), and expression constructs if $\,\langle$ exp $\rangle\,$ then $\,\langle$ exp $\rangle\,$ else $\,\langle$ exp $\rangle\,$ and others associates weakly than weakest infix operators (those that are declared with infix 0.