Shell function definition: why is there a space after the opening brace?

Question

You probably know of that fork bomb:

 :(){ :|:&};: #WARNING: harmful code

I wonder why it is necessary, for it to parse, to include a space after the opening curly brace.

Thanks.

Answer 1

{}

          { list; }

Placing a list of commands between curly braces causes the list to be executed in the current shell context. No subshell is created. The semicolon (or newline) following list is required.

In addition to the creation of a subshell, there is a subtle difference between these two constructs due to historical reasons. The braces are reserved words, so they must be separated from the list by blanks or other shell metacharacters. The parentheses are operators, and are recognized as separate tokens by the shell even if they are not separated from the list by whitespace.

// source

Answer 2

I think @rush may be giving a correct if misleading answer here. The fork-bomb defines a function called ":". The code in the curly braces is not executed until the function is called by the final ":". So the curly braces as command grouping and the curly braces as function body are syntactically the same but have different semantics.
From the same document as @rush cites:

Note that for historical reasons, in the most common usage the curly braces that surround the body of the function must be separated from the body by blanks or newlines. This is because the braces are reserved words and are only recognized as such when they are separated from the command list by whitespace or another shell metacharacter. Also, when using the braces, the list must be terminated by a semicolon, a ‘&’, or a newline.