OpenGL ES Shading Language - Praise for OpenGL ES 30 Programming Guide - page 103

Just as in C or C++, a variable that is declared as const is read-only and
cannot be modified within the source.
Structures
In addition to using the basic types provided in the language, it is possible
to aggregate variables into structures much like in C. The declaration
syntax for a structure in the OpenGL ES Shading Language is shown in the
following example:
struct fogStruct
{
vec4 color;
float start;
float end;
} fogVar;
The preceding definition will result in a new user type named fogStruct
and a new variable named fogVar .
Structures can be initialized using constructors. After a new structure type
is defined, a new structure constructor is also defined with the same name
as the type. There must be a one-to-one correspondence between types
in the structure and those in the constructor. For example, the preceding
structure could be initialized using the following construction syntax:
struct fogStruct
{
vec4 color;
float start;
float end;
} fogVar;
fogVar = fogStruct(vec4(0.0, 1.0, 0.0, 0.0), // color
0.5, // start
2.0); // end
The constructor for the structure is based on the name of the type and
takes as arguments each of the components. Accessing the elements of a
structure is done just as you would with a structure in C, as shown in the
following example:
vec4 color = fogVar.color;
float start = fogVar.start;
float end = fogVar.end;