DFL Format Reference

DFL (Defeasible Logic Format) is a textual syntax for writing defeasible logic theories.

File Extension

.dfl

Comments

Lines starting with # are comments:

# This is a comment
f1: >> bird   # Inline comment

Grammar

theory      = (rule | superiority | comment | blank)*
rule        = label ":" body? arrow head
superiority = label ">" label
label       = identifier
body        = literal ("," literal)*
head        = literal
literal     = negation? identifier
negation    = "-" | "~" | "¬"
arrow       = ">>" | "->" | "=>" | "~>"
identifier  = [a-zA-Z_][a-zA-Z0-9_-]*
comment     = "#" .*
blank       = whitespace

Rules

Facts (>>)

No body, unconditional truth.

f1: >> bird
f2: >> penguin
f3: >> -guilty

Strict Rules (->)

Body implies head necessarily.

r1: penguin -> bird
r2: human, mortal -> dies
r3: a, b, c -> result

Defeasible Rules (=>)

Body typically implies head.

r1: bird => flies
r2: penguin => -flies
r3: student, employed => busy

Defeaters (~>)

Body blocks head (doesn’t prove it).

d1: broken_wing ~> flies
d2: exception ~> normal_behavior

Literals

Simple Literals

bird
flies
has_feathers

Negated Literals

All three forms are equivalent:

-flies
~flies
¬flies

Identifiers

Valid identifiers:

• Start with letter or underscore
• Contain letters, digits, underscores, hyphens

bird
_private
has-feathers
rule_1

Rule Bodies

Single Literal

r1: bird => flies

Multiple Literals (Conjunction)

Comma-separated:

r1: bird, healthy => flies
r2: a, b, c, d => result

Mixed Negation

r1: bird, -penguin => flies
r2: student, -rich, -employed => needs_loan

Superiority Relations

Declare that one rule beats another:

r2 > r1
more_specific > general
exception > default

Multiple declarations:

r3 > r2
r2 > r1

Complete Example

# The Penguin Example
# Demonstrates defeasible reasoning with exceptions

# Facts
f1: >> bird
f2: >> penguin

# Strict rule: all penguins are birds
s1: penguin -> bird

# Defeasible rules
r1: bird => flies
r2: bird => has_feathers
r3: penguin => -flies
r4: penguin => swims

# Superiority: penguin rules beat bird rules
r3 > r1

# Defeater: broken wings block flying
d1: broken_wing ~> flies

Whitespace

• Leading/trailing whitespace is ignored
• Whitespace around operators is optional
• Blank lines are ignored

These are equivalent:

r1: bird => flies
r1:bird=>flies
r1 : bird => flies

Error Handling

Common errors:

Best Practices

1. Use meaningful labels

   # Good
   bird_flies: bird => flies
   
   # Avoid
   r1: bird => flies
   

2. Group related rules

   # Bird rules
   r1: bird => flies
   r2: bird => has_feathers
   
   # Penguin rules
   r3: penguin => -flies
   

3. Document superiority

   # Penguin is more specific than bird
   r3 > r1