Syntax

Postfix application (`//`)

expr // f is equivalent to f[expr]. The // operator has the lowest precedence, so it applies to the entire left-hand expression.

$ wo '4 // Sqrt'
2

$ wo '{1, 2, 3} // Length'
3

Postfix after an operator chain:

$ wo '1 + 2 // ToString'
3

$ wo 'Sqrt /@ {1, 4, 9} // Length'
3

Postfix with a curried function call:

$ wo '{1, 4, 9} // Map[Sqrt]'
{1, 2, 3}

Chained postfix:

$ wo '16 // Sqrt // Sqrt'
2

@ has higher precedence than ==, so f@x == y means (f@x) == y.

$ wo 'Length@{1,2,3} == 3'
True

$ wo 'Length@Union@{1,2,1} == Length@Union[{1,2,1} + Range@3] == 3'
False

-> creates a rule, and :> creates a delayed rule. These can be used anywhere in expressions, not just in replacement contexts.

$ wo '{1, 2} -> 3'
{1, 2} -> 3

$ wo 'Frame -> All'
Frame -> All

$ wo 'x :> x + 1'
x :> x + 1

expr[[i, j]] extracts nested parts: first part i, then part j from the result.

$ wo 'x = {{a,b},{c,d}}; x[[2,1]]'
c

$ wo 'x = {{a,b},{c,d}}; x[[1,2]]'
b

$ wo 'FullForm[a[[1,2,3]]]'

Part::partd: Part specification a[[1,2,3]] is longer than depth of object.
Part[a, 1, 2, 3]

x++ increments x by 1 and returns the old value. x-- decrements x by 1 and returns the old value.

$ wo 'x = 5; x++; x'
6

$ wo 'x = 10; x--; x'
9

$ wo 'x = 0; Do[x++, {i, 1, 5}]; x'
5

Assigning to a Part expression modifies the list in-place.

$ wo 'x = {1, 2, 3}; x[[2]] = 99; x'
{1, 99, 3}

$ wo 'x = {{1, 2}, {3, 4}}; x[[2, 1]] = 99; x'
{{1, 2}, {99, 4}}

$ wo 'x = {a, b, c}; x[[3]] = z; x'
{a, b, z}

Both sides of -> (Rule) are evaluated.

$ wo '{2, First@{1}} -> "Q"'
{2, 1} -> Q

Anonymous functions can be stored in variables and called later.

$ wo 'f = (# + 1) &; f[5]'
6

$ wo 'g = (# * #2) &; g[3, 4]'
12

$ wo 'f = # &; Map[f, {1, 2, 3}]'
{1, 2, 3}

Slots (#) bind to the innermost & (Function).

$ wo '(# + 1 &) /@ {10, 20, 30}'
{11, 21, 31}

The | operator represents alternatives in pattern matching.

$ wo 'Cases[{1, "a", 2, "b"}, _Integer]'
{1, 2}

$ wo 'Cases[{1, 2, 3, 4, 5}, Except[2 | 4]]'
{1, 3, 5}