An identifier is a symbol declared as a name for a variable, type, procedure, etc. The region of the program over which a declaration applies is called the scope of the declaration. Scopes can be nested. The meaning of an identifier is determined by the smallest enclosing scope in which the identifier is declared unless overloading resolution rules suggest otherwise.

An expression specifies a computation that produces a value or location. Expressions that produce locations are called l-values. An l-value can denote either a location or the value the location contains, depending on the context.

A Nim program consists of one or more text source files containing Nim code. It is processed by a Nim compiler into an executable. The nature of this executable depends on the compiler implementation; it may, for example, be a native binary or JavaScript source code.

The compiler parses Nim source code into an internal data structure called the abstract syntax tree (AST). Then, before executing the code or compiling it into the executable, it transforms the AST through semantic analysis. This adds semantic information such as expression types, identifier meanings, and in some cases expression values. An error detected during semantic analysis is called a static error. Errors described in this manual are static errors when not otherwise specified.

A panic is an error that the implementation detects and reports at runtime. The method for reporting such errors is via raising exceptions or dying with a fatal error. However, the implementation provides a means to disable these runtime checks. See the section pragmas for details.

Whether a panic results in an exception or in a fatal error is implementation specific. Thus the following program is invalid; even though the code purports to catch the IndexDefect from an out-of-bounds array access, the compiler may instead choose to allow the program to die with a fatal error.

An unchecked runtime error is an error that is not guaranteed to be detected, and can cause the subsequent behavior of the computation to be arbitrary. Unchecked runtime errors cannot occur if only safe language features are used and if no runtime checks are disabled.

A constant expression is an expression whose value can be computed during semantic analysis of the code in which it appears. It is never an l-value and never has side effects. Constant expressions are not limited to the capabilities of semantic analysis, such as constant folding; they can use all Nim language features that are supported for compile-time execution. Since constant expressions can be used as an input to semantic analysis (such as for defining array bounds), this flexibility requires the compiler to interleave semantic analysis and compile-time code execution.

It is mostly accurate to picture semantic analysis proceeding top to bottom and left to right in the source code, with compile-time code execution interleaved when necessary to compute values that are required for subsequent semantic analysis. We will see much later in this document that macro invocation not only requires this interleaving, but also creates a situation where semantic analysis does not entirely proceed top to bottom and left to right.