34.9 White space

Stan allows spaces between elements of a program. The white space characters allowed in Stan programs include the space (ASCII 0x20), line feed (ASCII 0x0A), carriage return (0x0D), and tab (0x09). Stan treats all whitespace characters interchangeably, with any sequence of whitespace characters being syntactically equivalent to a single space character. Nevertheless, effective use of whitespace is the key to good program layout.

Line breaks between statements and declarations

Each statement of a program should appear on its own line. Declaring multiple variables of the same type can be accomplished in a single statement with the syntax

real mu, sigma;

No tabs

Stan programs should not contain tab characters. Using tabs to layout a program is highly unportable because the number of spaces represented by a single tab character varies depending on which program is doing the rendering and how it is configured.

Two-character indents

Stan has standardized on two space characters of indentation, which is the standard convention for C/C++ code.

34.9.1 Space between if, { and condition

Use a space after ifs. For instance, use if (x < y) {..., not if(x < y){ ....

No space for function calls

There should not be space between a function name and the arguments it applies to. For instance, use normal(0, 1), not normal (0,1).

Spaces around operators

There should be spaces around binary operators. For instance, use y[1] = x, not y[1]=x, use (x + y) * z not (x+y)*z.

Unary operators are written without a space, such as in -x, !y.

No spaces in type constraints

Another exception to the above rule is when the assignment operator (=) is used inside a type constraint, such as

real<lower=0> x;

Spaces should still be used in arithmetic and following commas, as in

real<lower=0, upper=a * x + b> x;

Breaking expressions across lines

Sometimes expressions are too long to fit on a single line. In that case, the recommended form is to break before an operator,48 aligning the operator to a term above to indicate scoping.
For example, use the following form

vector[J] p_distance = Phi((distance_tolerance - overshot)
                           ./ ((x + overshot) * sigma_distance))
                       - Phi((-overshot)
                             ./ ((x + overshot) * sigma_distance));

Here, the elementwise division operator (./) is aligned to clearly signal the division is occuring inside the parethenesis, while the subtraction indicates it is between the function applications (Phi).

For functions with multiple arguments, break after a comma and line the next argument up underneath as follows.

y[n] ~ normal(alpha + beta * x + gamma * y,
              pow(tau,-0.5));

Spaces after commas

Commas should always be followed by spaces, including in function arguments, sequence literals, between variable declarations, etc.

For example,

normal(alpha * x[n] + beta, sigma);

is preferred over

normal(alpha * x[n] + beta,sigma);

Unix newlines

Wherever possible, Stan programs should use a single line feed character to separate lines. All of the Stan developers (so far, at least) work on Unix-like operating systems and using a standard newline makes the programs easier for us to read and share.

Platform specificity of newlines

Newlines are signaled in Unix-like operating systems such as Linux and Mac OS X with a single line-feed (LF) character (ASCII code point 0x0A). Newlines are signaled in Windows using two characters, a carriage return (CR) character (ASCII code point 0x0D) followed by a line-feed (LF) character.


  1. This is the usual convention in both typesetting and other programming languages. Neither R nor BUGS allows breaks before an operator because they allow newlines to signal the end of an expression or statement.↩︎