Learn

Pascal has rich type definitions. Group them in a type block before var.

Enumerations

type
    TDay = (Mon, Tue, Wed, Thu, Fri, Sat, Sun);
    TColor = (Red, Green, Blue);

var
    today: TDay;
begin
    today := Wed;
    WriteLn(Ord(today));            { 2 — index in declaration order }
end.

Enumerations are ordinal — you can use them in case, for ... to, and set of:

for d := Mon to Fri do ...

case today of
    Mon..Fri: WriteLn('weekday');
    Sat, Sun: WriteLn('weekend');
end;

weekdays: set of TDay = [Mon..Fri];

Successor / predecessor:

Succ(Mon)        { Tue }
Pred(Wed)        { Tue }

Subranges

A subrange of an ordinal type — runtime-checked when range checking is on:

type
    TPercent = 0..100;
    TLetter = 'a'..'z';

var grade: TPercent;
begin
    grade := 95;       { OK }
    { grade := 200; }  { compile error or runtime error }
end.

Type aliases

Give a friendly name to existing types:

type
    TUserID = integer;
    TName = string;
    TIntArray = array of integer;

var id: TUserID;

The compiler treats TUserID and integer as compatible. For stricter types (incompatible aliases), use type integer:

type
    TMeters = type integer;     { distinct — won't auto-convert from integer }

Constants

const
    MAX_USERS = 1000;
    PI = 3.14159;
    GREETING = 'Hello, World!';

Typed constants — initialized variables that act like constants:

const
    days: array[1..7] of string = ('Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun');
    config: record
        host: string;
        port: integer;
    end = (host: 'localhost'; port: 8080);

Typedefs in practice

Real Pascal code uses types extensively to:

• Document intent (TPercent not integer)
• Catch mistakes (subranges, distinct types)
• Enable polymorphism (class hierarchies)
• Enable generics (TList<TUser>)

This is what made Pascal a great teaching language — types make programs self-documenting and compiler-checked.