Is there a summary of answers Head[] can give?

Question

Sometimes you need to check for the type of an expression. You use Head[data] to get answers.

Some data:

data = {7, 1.4, Red, "hp", hps};
Head /@ data
(*{Integer, Real, RGBColor, String, Symbol}*)

Is there summary of what kind of answers Head[] can give?

Answer 1

Head can return any head. There is no predefined list.

expr = myArbitraryHead[1, 2, 3];

Head[expr]

myArbitraryHead

A head does not even need to be a Symbol:

expr2 = (2 Pi)[x, y, z];

Head[expr2]

2 π

Most heads are shown explicitly in the FullForm of the expression:

FullForm[{"a" + "b", 1/3}]

Head /@ {"a" + "b", 1/3}

List[Plus["a", "b"], Rational[1, 3]]
{Plus, Rational}

Some heads are implicit, such as in the atomic objects String, Integer, and Symbol:

FullForm[{"a", 1, Pi}]

Head /@ {"a", 1, Pi}

List["a", 1, Pi]
{String, Integer, Symbol}

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As a side note the head of an expression is also returned when you ask for Part zero:

3[[0]]

Integer

Because Part returns sequences wrapped in the original head of the expression you get a strange result if extracting {0}:

3[[{0}]]

Integer[Integer]