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Notable Differences from Other Languages

:= vs <-

The familiar assignment operator from C, =, splits into two forms in plover: :=, <-. In short, := is used to declare and initialize a variable (whose type will generally be inferred), while <- is used to mutate a variable already in scope.

See definitions [1] and locations [2].

Value of a Block

As described in sequencing [3], the value of a sequence expression (a ; b) is the value of b. A common error is to call a function as the last statement of a block which is intended to have void type.

For instance,

f (x :: int) :: () := (
  printf "x: %d" x;

will produce an error, because the (often ignored) return type from printf is actually int. The simplest fix in this case is to add a void literal, ():

f (x :: int) :: () := (
  printf "x: %d" x; ();

Another consequence of this convention is that the return keyword is optional on the last statement of a function. It is still useful as an early return mechanism, of course.


The branches of an if expression must have the same type. When the else branch is omitted, it has void type. Hence

if condition then printf "hi there\n";

is incorrect.

Notably, when return is used, the expression being returned is checked against the return type of the enclosing function, but the type of the return expression itself changes to match what is expected by its immediate context. Thus, the following is ok:

f (x :: int) :: int := (
  if x > 0 then return x;
  return -x;

but, when possible, this is a better style:

f (x :: int) :: int := if x > 0 then x else -x;



A block is typically delimited with parentheses: (a; b; ...). Semicolons (;) must separate statements within a block. Top-level definitions and extern definitions are, likewise, separated by semicolons.


In the face of a troubling parsing or typechecking error, make sure you haven't left out any semicolons.

Vectors and Matrices

The size of a vector object is represented, internally, as a list of Plover expressions. Each position in this list is called an "index", and the expression at a given index is called a "bound" or a "dimension." This is meant to coincide with the usual language for discussing tensors. We will refer to the number of indices as the object's rank.

Generally we call rank-1 and rank-2 objects vectors and matrices, respectively.

Plover overloads multiplication [4] to handle certain frequent use cases. In particular, for a matrix-vector product (M*v), the vector is interpreted as a column vector. The vector-matrix product is not implemented; instead, you may use (v^T*M). Note that the builtin transpose operator converts a rank-1 vector of dimension n into an explicit, rank-2, [1,n] matrix. For this reason, taking a transpose twice may not return an object identical to the initial one. Some discussion on the complexity of this issue: julia [7].


Plover indexing is 0-based. Matrices are row-major. See ranges [5] and slicing [6] for more information.