Language overview

Basic features

Feature	Example
Single-line comment	// This is a single line comment
Multi-line or intervening comment	/* This is a multiline comment, when more text is needed */
Convert integer to string	int2string(n)
Convert real to string	real2string(pi)
Convert boolean to string	bool2string(true)
Convert integer to real	int2real(n)
Convert real to integer	real2int(pi)
Terminate statements with semi-colon ;	printLn("Hello, world");

Printing and debugging

Feature	Example
Print something (real/integer/string) to debug	debug(something)
Printing and adding a new line	printLn(text);
Printing without adding a new line	print(text);
Print only an empty line	print("\n");

• All types must be converted to string before printing, unless using debug.
• For reals, only a few decimals will be retained when printed.

Functions

Feature	Example
Creating a function without a return	function flip(datapoint: Bool, probability: Real) { observe datapoint ~ Bernoulli(probability); };
Creating a function with a return	function flip(datapoint: Bool, probability: Real) => Real { observe datapoint ~ Bernoulli(probability); return probability; }
Conditional (if/else statement)	if (condition) { execute_when_true } else { execute_when_false }
Loop (for statement)	for i in 1 to n { ... }

• A function needs to specify its input parameters and their types.
• A function can have zero input parameters.
• A function needs to specify its return parameters and their types, if there is a return.
• A function may return nothing via return; or have no return statement at all.
• The return statement may appear within a for or an if.
• The only effects that a function can have are changing the likelihood, printing things, and returning an output.
• Looping allows for limited side-effects.
• Unlike some other languages, the curly braces are mandatory in if statements (the parentheses around the condition are not).

Working with scalar data

Feature	Example
The basic datatypes are integer, real, boolean, and string	42 (Int), 3.14 (Real), true or false (Bool), "hello" (String)
Arithmetic on numbers	2.0 + 3.0, 2.0 - 3.0, 2.0 * 3.0, 2.0 / 3.0
Comparisons (return booleans)	2.0 < 3.0, 2.0 <= 3.0, 2.0 == 3.0, 2.0 >= 3.0, 2.0 > 3.0
Boolean logic, AND (true if both are true)	a && b
Boolean logic, OR (true if one is true)	a || b
Boolean logic, NOT (negates)	!a
Explicit numeric conversion before comparing	Real(n) > 5.0

Working with sequences and matrices

Feature	Example
Joining with paste0 (no separator)	paste0(["espresso", "macchiato"]) returns "espressomacchiato"
Joining with paste (defining separator)	paste(["espresso", "macchiato"], " ") returns "espresso macchiato"
Creating a new sequence	let testSeq = [1, 2]; will create a sequence of integers with length 2
Getting the i-th element of the sequence (sequences are 1-indexed)	testSeq[1]
Getting the length of a sequence	length(testSeq)
Applying a function to each element of a sequence	let strSeq = apply(bool2string, boolSeq);
Creating a new matrix	let testMat = mtxCreate(2, 2, [1, 2, 3, 4]); creates a matrix with dimensions 2x2

• The basic datatypes are integer (Int), real (Real), boolean (Bool), and string (String). Each of them can be converted to a sequence by []. For instance, Int[] for a sequence of integers.
• It is not possible to change individual elements. For example: let testSeq[1] = x; // compiler error!

Probabilistic programming

Feature	Example
A model function is needed by every program. It specifies the probabilistic model.	model function coinModel(coinflips: Bool[]) => Real[] { // specify prior // condition the likelihood on data // return posterior };
Use the assume keyword to sample form a prior distribution (here using the Beta distribution)	assume p ~ Beta(a, b);
Use the observe keyword to condition the likelihood on observed data (here using the Beta distribution)	observe data ~ Beta(a, b);
To manipulate the likelihood directly use weight	weight(lik);
or logWeight on the logarithmic scale	logWeight(lik);
The posterior is the returned value of the model function over one or more parameters	return p;