Fix implication formatting

src/format/formulas.rs

@@ -153,15 +153,74 @@ impl<'formula> std::fmt::Debug for FormulaDisplay<'formula>
 			},
 			crate::Formula::Implies(crate::Implies{direction, antecedent, implication}) =>
 			{
+				let antecedent_requires_parentheses = match **antecedent
+				{
+					// Implication is right-associative and thus requires parentheses when nested on
+					// the antecedent side
+					crate::Formula::Implies(_) => true,
+					_ => false,
+				};
+
+				let implication_requires_parentheses = match **antecedent
+				{
+					// Implication is right-associative and thus doesn’t require parentheses when
+					// nested on the implication side with the same implication direction
+					crate::Formula::Implies(crate::Implies{direction: nested_direction, ..})
+						if nested_direction == *direction => false,
+					// If the nested implication is in the other direction, parentheses are needed
+					crate::Formula::Implies(_) => true,
+					_ => false,
+				};
+
+				let format_antecedent = |format: &mut std::fmt::Formatter| -> Result<_, _>
+				{
+					if antecedent_requires_parentheses
+					{
+						write!(format, "(")?;
+					}
+
+					write!(format, "{:?}", display_formula(antecedent, precedence))?;
+
+					if antecedent_requires_parentheses
+					{
+						write!(format, ")")?;
+					}
+
+					Ok(())
+				};
+
+				let format_implication = |format: &mut std::fmt::Formatter| -> Result<_, _>
+				{
+					if implication_requires_parentheses
+					{
+						write!(format, "(")?;
+					}
+
+					write!(format, "{:?}", display_formula(implication, precedence))?;
+
+					if implication_requires_parentheses
+					{
+						write!(format, ")")?;
+					}
+
+					Ok(())
+				};
+
 				match direction
 				{
 					crate::ImplicationDirection::LeftToRight =>
-						write!(format, "{:?} -> {:?}", display_formula(antecedent, precedence),
-							display_formula(implication, precedence))?,
+					{
+						format_antecedent(format)?;
+						write!(format, " -> ")?;
+						format_implication(format)?;
+					},
 					crate::ImplicationDirection::RightToLeft =>
-						write!(format, "{:?} <- {:?}", display_formula(implication, precedence),
-							display_formula(antecedent, precedence))?,
-				};
+					{
+						format_implication(format)?;
+						write!(format, " <- ")?;
+						format_antecedent(format)?;
+					},
+				}
 			},
 			crate::Formula::IfAndOnlyIf(arguments) =>
 			{