1. Data Types

PostScript has a rich type system with distinct types for different kinds of data. Understanding the type system is essential for writing robust PostScript programs and avoiding type-related errors.

1.1. Overview

Every PostScript object has a type that determines what operations can be performed on it. The type system includes simple types (stored by value) and composite types (stored by reference), with clear rules for type checking, conversion, and compatibility.

1.2. Type Hierarchy

PostScript’s type system organizes types into a clear hierarchy:

Complete type hierarchy
PostScript Types
├── Simple Types (stored by value)
│   ├── Numeric Types
│   │   ├── integertype         Integer numbers
│   │   └── realtype            Floating-point numbers
│   ├── Boolean Types
│   │   └── booleantype         True/false values
│   ├── Name Types
│   │   └── nametype            Identifiers
│   ├── Operator Types
│   │   └── operatortype        Built-in operations
│   ├── Mark Types
│   │   └── marktype            Stack delimiter
│   ├── Null Types
│   │   └── nulltype            Empty value
│   └── Save Types
│       └── savetype            VM snapshot
└── Composite Types (stored by reference)
    ├── Collection Types
    │   ├── stringtype          Byte sequences
    │   ├── arraytype           Object arrays
    │   ├── packedarraytype     Compact arrays (Level 2+)
    │   └── dicttype            Key-value maps
    ├── I/O Types
    │   └── filetype            Input/output streams
    ├── Graphics Types
    │   ├── fonttype            Font dictionaries
    │   └── gstatetype          Graphics state (Level 2+)
    └── Condition Types
        └── conditiontype       Synchronization (Level 2+)

1.3. Simple Types

Simple types store their values directly in the object.

1.3.1. Integer Type

Whole numbers within implementation-defined range.

Table 1. Integer characteristics
Property Description

Type Name

integertype

Range

Typically -2³¹ to 2³¹-1 (32-bit signed)

Literal Syntax

42, -17, 0, +100

Radix Notation

8#77, 16#FF, 2#1010

Storage

By value

Mutability

Immutable
Integer examples
42 type                    % Returns /integertype
-17 type                   % Returns /integertype
16#FF type                 % Returns /integertype

% Type checking
42 dup type /integertype eq  % Returns true

% Operations
10 20 add                  % 30 (integer)
10 3 div                   % 3.33... (becomes real)
10 3 idiv                  % 3 (stays integer)

1.3.2. Real Type

Floating-point numbers with fractional parts.

Table 2. Real characteristics
Property Description

Type Name

realtype

Range

Implementation-dependent (typically IEEE 754)

Precision

Implementation-dependent (typically single or double)

Literal Syntax

3.14, -2.5, 1.0, .5, 6.02e23

Storage

By value

Mutability

Immutable
Real examples
3.14 type                  % Returns /realtype
-2.5 type                  % Returns /realtype
6.02e23 type               % Returns /realtype

% Type checking
3.14 dup type /realtype eq % Returns true

% Operations
3.14 2.0 mul               % 6.28 (real)
10 3.0 div                 % 3.33... (real)
1.5 ceiling                % 2.0 (real result)

1.3.3. Boolean Type

Logical true/false values.

Table 3. Boolean characteristics
Property Description

Type Name

booleantype

Values

true, false

Literal Syntax

true or false

Storage

By value

Mutability

Immutable
Boolean examples
true type                  % Returns /booleantype
false type                 % Returns /booleantype

% Type checking
true dup type /booleantype eq  % Returns true

% Operations
true false and             % false
true false or              % true
true not                   % false
5 3 gt                     % true

1.3.4. Name Type

Identifiers used as dictionary keys and references.

Table 4. Name characteristics
Property Description

Type Name

nametype

Literal Syntax

/name (literal), name (executable)

Storage

By value (internal integer ID)

Mutability

Immutable

Max Length

128 chars (Level 1), 65535 (Level 2+)
Name examples
/myname type               % Returns /nametype
/add type                  % Returns /nametype

% Type checking
/x dup type /nametype eq   % Returns true

% Name operations
/myname ==                 % Prints /myname
(string) cvn type          % Returns /nametype
/name 20 string cvs        % Returns (name)

1.3.5. Operator Type

Built-in primitive operations.

Table 5. Operator characteristics
Property Description

Type Name

operatortype

Examples

add, def, moveto

Storage

By value (reference to built-in)

Mutability

Immutable

Executable

Always executable
Operator examples
/add load type             % Returns /operatortype
/moveto load type          % Returns /operatortype

% Type checking
/add load dup type /operatortype eq  % Returns true

% Cannot be made literal
/add load cvlit type       % Still /operatortype

1.3.6. Mark Type

Special stack marker object.

Table 6. Mark characteristics
Property Description

Type Name

marktype

Literal Syntax

mark or [

Storage

By value

Purpose

Stack delimiter for array construction
Mark examples
mark type                  % Returns /marktype
[ type                     % Returns /marktype

% Usage
mark 1 2 3 counttomark     % Returns 3
mark 1 2 3 ]              % Creates [1 2 3]

1.3.7. Null Type

Empty or undefined value.

Table 7. Null characteristics
Property Description

Type Name

nulltype

Literal Syntax

null

Storage

By value

Purpose

Represent "no value" or initialization
Null examples
null type                  % Returns /nulltype

% Type checking
null dup type /nulltype eq % Returns true

% Usage
/uninitialized null def
uninitialized null eq      % Returns true

1.3.8. Save Type

VM snapshot for save/restore operations.

Table 8. Save characteristics
Property Description

Type Name

savetype

Creation

save operator

Storage

By value (reference to snapshot)

Purpose

Mark VM state for restoration
Save examples
save type                  % Returns /savetype

% Usage
save
/temp 42 def
restore                    % temp is undefined again

1.4. Composite Types

Composite types store their values indirectly through references.

1.4.1. String Type

Sequences of 8-bit bytes.

Table 9. String characteristics
Property Description

Type Name

stringtype

Literal Syntax

(text) or <hex>

Max Length

65,535 bytes

Storage

By reference

Mutability

Mutable

Indexed Access

0-based byte access
String examples
(Hello) type               % Returns /stringtype
<4142> type                % Returns /stringtype

% Type checking
(text) dup type /stringtype eq  % Returns true

% Operations
(Hello) length             % 5
(Hello) 0 get             % 72 (ASCII 'H')
(Hello) dup 0 88 put      % Modifies to (Xello)

1.4.2. Array Type

Ordered collections of objects.

Table 10. Array characteristics
Property Description

Type Name

arraytype

Literal Syntax

[…​] (literal), {…​} (executable)

Max Length

65,535 elements

Storage

By reference

Mutability

Mutable

Indexed Access

0-based element access
Array examples
[1 2 3] type               % Returns /arraytype
{ 1 2 add } type           % Returns /arraytype (executable)

% Type checking
[1 2] dup type /arraytype eq  % Returns true

% Operations
[1 2 3] length            % 3
[1 2 3] 1 get            % 2
[1 2 3] dup 0 99 put     % Modifies to [99 2 3]

1.4.3. Packed Array Type (Level 2+)

Space-efficient read-only arrays.

Table 11. Packed array characteristics
Property Description

Type Name

packedarraytype

Creation

packedarray operator

Storage

By reference (compact)

Mutability

Read-only

Space

More compact than regular arrays
Packed array examples
mark 1 2 3 ] packedarray type  % Returns /packedarraytype

% Read-only
mark 1 2 3 ] packedarray
dup 0 99 put                    % Error: invalidaccess

1.4.4. Dictionary Type

Associative arrays (key-value pairs).

Table 12. Dictionary characteristics
Property Description

Type Name

dicttype

Creation

dict or << …​ >>

Storage

By reference

Mutability

Mutable

Key Types

Any object (typically names)
Dictionary examples
10 dict type               % Returns /dicttype
<< /a 1 >> type           % Returns /dicttype

% Type checking
10 dict dup type /dicttype eq  % Returns true

% Operations
<< /x 42 >> /x get        % 42
<< >> dup /y 10 put       % Add entry

1.4.5. File Type

Input/output streams.

Table 13. File characteristics
Property Description

Type Name

filetype

Creation

file operator, stdin, stdout

Storage

By reference

Mutability

State changes

Purpose

Reading/writing data
File examples
(file.txt) (r) file type   % Returns /filetype
stdin type                  % Returns /filetype

% Type checking
stdin dup type /filetype eq % Returns true

1.4.6. Font Type

Font dictionaries.

Table 14. Font characteristics
Property Description

Type Name

fonttype

Creation

definefont, findfont

Storage

By reference (special dictionary)

Mutability

Varies

Purpose

Text rendering
Font examples
/Times-Roman findfont type % Returns /fonttype

% Type checking
currentfont type            % Returns /fonttype

1.4.7. Graphics State Type (Level 2+)

Captured graphics state.

Table 15. GState characteristics
Property Description

Type Name

gstatetype

Creation

gstate operator

Storage

By reference

Purpose

Save/restore graphics state
GState examples
gstate type                % Returns /gstatetype

% Usage
gstate /mygs exch def
mygs setgstate

1.5. Type Checking

1.5.1. type Operator

Returns the type name of an object:

Getting object types
42 type                    % /integertype
3.14 type                  % /realtype
(text) type                % /stringtype
[1 2 3] type              % /arraytype
true type                  % /booleantype
/name type                 % /nametype

1.5.2. Type Comparison

Checking specific types
% Check if integer
/CheckInteger {
    type /integertype eq
} def

42 CheckInteger            % true
3.14 CheckInteger          % false

% Check if numeric
/CheckNumeric {
    type dup /integertype eq
    exch /realtype eq or
} def

42 CheckNumeric            % true
3.14 CheckNumeric          % true
(text) CheckNumeric        % false

1.5.3. Type Validation

Safe type checking
/RequireType {
    % in: object expected-type
    % out: object (or error)

    1 index type 1 index ne {
        /typecheck cvx signalerror
    } {
        pop
    } ifelse
} def

% Usage
42 /integertype RequireType     % OK
(text) /integertype RequireType % Error

1.6. Type Conversion

1.6.1. Numeric Conversions

Operator Conversion Example

cvi

To integer

3.14 cvi → 3

cvr

To real

42 cvr → 42.0

round

Round to nearest

3.7 round → 4.0

truncate

Truncate to integer

3.7 truncate → 3.0

floor

Round down

3.7 floor → 3.0

ceiling

Round up

3.2 ceiling → 4.0
Numeric conversion examples
% Real to integer
3.14 cvi                   % 3
-2.8 cvi                   % -2

% Integer to real
42 cvr                     % 42.0

% Rounding
3.7 round cvi              % 4
3.2 floor cvi              % 3
3.2 ceiling cvi            % 4

1.6.2. String Conversions

Operator Conversion Example

cvs

To string

42 10 string cvs → (42)

cvn

String to name

(name) cvn → /name

cvi

String to integer

(42) cvi → 42

cvr

String to real

(3.14) cvr → 3.14
String conversion examples
% Number to string
42 10 string cvs           % (42)
3.14 20 string cvs         % (3.14)

% String to number
(42) cvi                   % 42
(3.14) cvr                 % 3.14

% String to name
(myname) cvn               % /myname

% Name to string
/myname 20 string cvs      % (myname)

1.6.3. Literal/Executable Conversion

Operator Conversion Example

cvlit

To literal

add cvlit → /add

cvx

To executable

/add cvx → add
Literal/executable examples
% Make literal
{ 1 2 add } cvlit          % Literal array
add cvlit                  % /add (name)

% Make executable
/add cvx                   % add (operator)
[1 2 /add] cvx            % Executable array

1.7. Type Compatibility

1.7.1. Numeric Type Mixing

Arithmetic operations automatically promote integers to reals when needed:

Automatic promotion
% Integer + Integer → Integer
10 20 add                  % 30 (integer)

% Integer + Real → Real
10 20.0 add               % 30.0 (real)

% Integer / Integer → Real
10 3 div                   % 3.33... (real)

% Integer idiv Integer → Integer
10 3 idiv                  % 3 (integer)

1.7.2. Type Promotion Rules

Promotion hierarchy
Integer → Real (when mixed with real)
Integer → Real (for division with div)
Integer → Integer (for idiv, mod)

1.7.3. Compatible Operations

Operations by type compatibility
% Numeric operations (int or real)
10 20 add mul div sub

% String operations
(abc) (def) eq
(text) length

% Array operations
[1 2 3] [4 5 6] eq
[1 2] length

% Dictionary operations
<< /a 1 >> /a known
10 dict length

1.8. Type-Specific Operations

1.8.1. Integer-Only Operations

Operator Description

idiv

Integer division

mod

Modulo (remainder)

bitshift

Bit shift

and (bitwise)

Bitwise AND

or (bitwise)

Bitwise OR

xor (bitwise)

Bitwise XOR

not (bitwise)

Bitwise NOT
Integer-specific examples
10 3 idiv                  % 3 (integer division)
10 3 mod                   % 1 (remainder)
4 2 bitshift               % 16 (left shift)
12 10 and                  % 8 (bitwise AND)

1.8.2. Real-Only Operations

Operator Description

sin

Sine (degrees)

cos

Cosine (degrees)

atan

Arctangent

sqrt

Square root

exp

Exponential

ln

Natural logarithm

log

Base-10 logarithm
Real-specific examples
90 sin                     % 1.0
0 cos                      % 1.0
1 atan                     % 45.0
9.0 sqrt                   % 3.0
1 exp                      % 2.71828...

1.8.3. String-Specific Operations

Operator Description

search

Find substring

anchorsearch

Search from start

token

Parse token from string

1.8.4. Array-Specific Operations

Operator Description

aload

Unpack array to stack

astore

Pack stack into array

packedarray

Create packed array

1.8.5. Dictionary-Specific Operations

Operator Description

begin

Push onto dict stack

end

Pop from dict stack

def

Define in current dict

where

Find dict containing key

1.9. Type Error Handling

1.9.1. Common Type Errors

Error Cause

typecheck

Wrong operand type for operation

rangecheck

Value out of valid range

stackunderflow

Not enough operands

invalidaccess

Access permission violation
Type error examples
% typecheck
(text) 5 add               % Error: can't add string

% rangecheck
-5 array                   % Error: negative size

% stackunderflow
add                        % Error: needs 2 operands

% invalidaccess
[1 2 3] readonly
dup 0 99 put              % Error: read-only

1.9.2. Type-Safe Programming

Defensive type checking
/SafeAdd {
    % in: a b
    % out: sum or error

    % Check both are numeric
    1 index type dup /integertype eq
    exch /realtype eq or
    1 index type dup /integertype eq
    exch /realtype eq or
    and {
        add
    } {
        (Type error: expected numbers) =
        pop pop 0
    } ifelse
} def

3 4 SafeAdd                % 7
3 (x) SafeAdd              % Error message, returns 0

1.10. Type System Best Practices

1.10.1. Type Documentation

Document expected types
/WellTyped {
    % in: int int string
    % out: array
    % Description: Creates array from parameters

    % ... implementation ...
} def

1.10.2. Type Validation

Validate inputs
/ValidateParams {
    % Validate all parameters are correct type
    % before proceeding with operation
} def

1.10.3. Type Conversion Strategy

Explicit vs implicit conversion
% IMPLICIT - may be unclear
/proc { add } def

% EXPLICIT - clearer intent
/proc {
    cvr exch cvr exch       % Ensure both are real
    add
} def

1.10.4. Type Hierarchies

Organize types logically
% Check if numeric
/IsNumeric {
    type dup /integertype eq
    exch /realtype eq or
} def

% Check if collection
/IsCollection {
    type dup /arraytype eq
    1 index /stringtype eq or
    exch /dicttype eq or
} def

1.11. Type System Summary

1.11.1. Type Categories

Category Storage Types

Simple

By value

integer, real, boolean, name, operator, mark, null, save

Composite

By reference

string, array, dict, file, font, gstate, packedarray

Numeric

By value

integer, real

Collection

By reference

string, array, packedarray, dict

1.11.2. Type Mutability

Type Mutable Notes

integer

No

Immutable value

real

No

Immutable value

boolean

No

Immutable value

name

No

Immutable value

string

Yes

Mutable bytes

array

Yes

Mutable elements

packedarray

No

Read-only

dict

Yes

Mutable entries

file

State

Mutable state

1.11.3. Type Checking Quick Reference

Essential type operations
% Get type
obj type                   % Returns type name

% Check type
obj type /typename eq      % Boolean result

% Validate type
obj type /typename ne {
    /typecheck cvx signalerror
} if

% Convert type
obj cvi                    % To integer
obj cvr                    % To real
obj cvs                    % To string
obj cvn                    % To name
obj cvlit                  % To literal
obj cvx                    % To executable

1.12. See Also

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