Equality Rules

There are rules for both definitionally equality and propositional equality

Rules for Definitionally Equality

The rules must be normalising so,

As an example, a rule for symmetry x=y -> y=x can't be a definitionally equality rule and so must be a propositional equality rule because:

Rules for Propositional Equality

The rules are used to prove equality so its harder to have some automatic algorithm and so the programmer needs to apply the rules as required to do the proof.


There seems to be at least 2 sorts of rules we can apply to equations:

If we substitute, in a term M all occurrences of
context variable x by a term N of the same type
as x, then the result is the same type - it will remain true.

Γright adjointN:σ Γ,x:σ, Δright adjointM
Γ,Δ[N/x]right adjointM[N/x]

example where:

  • N= y+1
  • M = a x² + b x + c

Substitution happens in function application (that is, when deconstructing a function.

See β-reduction in λ-calulus.

(λ x:σ.N) M

Definition of 'subst'.

subst takes T x to T y.

We can think of T x as being an expression in x. So subst takes an expression in x to an expression in y.

right adjointq:(x=y)right adjointT : A -> Typeright adjointt :T x
subst q T t : T y

Below is the computational rule where the two values are already definitionaly equal. Here refl (propositional equality) is embedded into this definitionaly equality.

(subst (refl x x) T t)=t : T x


Also we can replace like for like in equations (is this substitution?).

Rewriting in Logic

Rewriting an expression into a logically equivalent one.

example: ¬A /\ ¬B -> ¬(A \/ B)

Computation rule

Refl disappears when properties are definitionaly equal.

subst (Refl P)=P


Substitution in Agda
id:{A: Set}->A->A
_==_:{A: Set}->A->A->Set
 subst:{A: Set} (C:A->Set) {x y:A}->x==y->C x->C y
subst : forall {x y} -> x == y -> x -> y
subst refl x = x


Substitution in Idris (replace)
||| Perform substitution in a term according to some equality.
replace : {a:_} -> {x:_} -> {y:_} -> {P : a -> Type} -> x = y -> P x -> P y
replace Refl prf = prf


Substitution as a fibration

example subst


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