Matlab Command-Function Duality

One reason MATLAB feels inconsistent is that it gives equal weight to both commands and functions. In particular, any function whose inputs are char vectors and that returns no outputs has an equivalent command syntax—the following two lines are equivalent:

• fun('argin_1',...,'argin_n')
• fun argin_1 ... argin_n

Note these peculiarities of the command syntax:

• The function name must be followed by at least one space
• It cannot be followed (no matter how many spaces apart) by a left parenthesis (; if there is one, MATLAB treats the call as function syntax
• It cannot contain the char-string single quote '
• The only argument separator is whitespace—exactly the opposite of function syntax
  • Therefore a char argument arg_i may not contain spaces
• In addition to chaining multiple commands on a single line with ;, the command syntax can also separate them with a comma ,

More importantly, this duality only applies to direct char vectors—the arguments may be neither variables nor any other data type. Command syntax effectively wraps each argument in single quotes ', as illustrated below:

>> disp "123"
"123"
>> a = 123;
>> disp a
a
>> a = '123';
>> disp a
a

Recognising Command Syntax

When variable naming is sloppy — say disp is a user-defined variable assigned the value 1 — the following statements are ambiguous:

• disp .* 1 actually yields ans = 1 — disp is treated as a variable
• disp .*1 actually yields '.*1 — disp is treated as a function

In general, given an identifier (which may name a function or a variable), MATLAB decides its kind from what follows:

• Followed by an assignment = — it is a variable

• Followed by parentheses () — disambiguated by Matlab Function Precedence Order (variable indexing vs. function call)

• Followed by a binary operator

  • If the operator has a trailing space, or no leading space, the identifier is a variable
  • If the operator has no trailing space but does have a leading space, the identifier is treated as the command syntax of a function

• This is one of the rare cases in which MATLAB is sensitive to whitespace.