A Hash is a generic collection of key-value pairs mapping keys of type K to values of type V.

Hashes are typically created with a hash literal denoted by curly braces ({ }) enclosing a list of pairs using => as delimiter between key and value and separated by commas ,.

{"one" => 1, "two" => 2}

Generic Type Argument

The generic type arguments for keys K and values V are inferred from the types of the keys or values inside the literal, respectively. When all have the same type, K/V equals to that. Otherwise it will be a union of all key types or value types respectively.

{1 => 2, 3 => 4}     # Hash(Int32, Int32)
{1 => 2, 'a' => 3}   # Hash(Int32 | Char, Int32)

Explicit types can be specified by immediately following the closing bracket with of (separated by whitespace), a key type (K) followed by => as delimiter and a value type (V). This overwrites the inferred types and can be used for example to create a hash that holds only some types initially but can accept other types as well.

Empty hash literals always need type specifications:

{} of Int32 => Int32 # => Hash(Int32, Int32).new

Hash-like Type Literal

Crystal supports an additional literal for hashes and hash-like types. It consists of the name of the type followed by a list of comma separated key-value pairs enclosed in curly braces ({}).

Hash{"one" => 1, "two" => 2}

This literal can be used with any type as long as it has an argless constructor and responds to []=.

HTTP::Headers{"foo" => "bar"}

For a non-generic type like HTTP::Headers, this is equivalent to:

headers = HTTP::Headers.new
headers["foo"] = "bar"

For a generic type, the generic types are inferred from the types of the keys and values in the same way as with the hash literal.

MyHash{"foo" => 1, "bar" => "baz"}

If MyHash is generic, the above is equivalent to this:

my_hash = MyHash(typeof("foo", "bar"), typeof(1, "baz")).new
my_hash["foo"] = 1
my_hash["bar"] = baz

The type arguments can be explicitly specified as part of the type name:

MyHash(String, String | Int32) {"foo" => "bar"}

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