# Why can't a left-recursive, non-deterministic, or ambiguous grammar be LL(1)?

I've learned from several sources that an LL(1) grammar is:

1. unambiguous,
2. not left-recursive,
3. and, deterministic (left-factorized).

What I can't fully understand is why the above is true for any LL(1) grammar. I know the LL(1) parsing table will have multiple entries at some cells, but what I really want to get is a formal and general (not with an example) proof to the following proposition(s):

A left-recursive (1), non-deterministic (2), or ambiguous (3) grammar is not LL(1).

• I don't know what left-factored' means, but Corollary 5.41 of Sippu & Soisalon-Soininen's Parsing Theory answers (1) and Theorem 5.37 of the same answers (3) for any SLL(k) grammar, while Lemma 8.39 shows that any LL(1) grammar is SLL(1). – Sylvain Jan 5 at 18:21
• @Sylvain Thanks for the references, and by 'left-factored' I mean 'deterministic grammar', e.g., S->aB|aC is not left-factored, S->aS' ;; S' -> B|C is left-factored). – MrGeek Jan 5 at 19:08

I have done some more research, and I think I've found a solution for the 1st and 2nd questions, as for the 3rd one, I found an existing solution on StackOverflow for it, the proof attempts are written below:

We start by writing the three rules of the definition of an LL(1) grammar:

For every production A -> α | β with α ≠ β:

1. FIRST(α) ∩ FIRST(β) = Ø.
2. If β =>* ε then FIRST(α) ∩ FOLLOW(A) = Ø (also, if α =>* ε then FIRST(β) ∩ FOLLOW(A) = Ø).
3. Including ε in rule (1) implies that at most one of α and β can derive ε.

Proposition 1: A non-factored grammar is not LL(1).

Proof:

If a grammar G is non-factored then there exists a production in G of the form:

A -> ωα1 | ωα2 | ... | ωαn


(where αi is the i-th α, not the symbols α and i), with α1 ≠ α2 ≠ ... ≠ αn. We can then easily show that:

∩(i=1,..,n) FIRST(ωαi) ≠ Ø


which contradicts rule (1) of the definition, thus, a non-factored grammar is not LL(1). ∎

Proposition 2: A left-recursive grammar is not LL(1).

Proof:

If a grammar is left-recursive then there exists a production in G of the form:

S -> Sα | β


Three cases arise here:

1. If FIRST(β) ≠ {ε} then:

    FIRST(β) ⊆ FIRST(S)

=>  FIRST(β) ∩ FIRST(Sα) ≠ Ø

which contradicts rule (1) of the definition.

2. If FIRST(β) = {ε} then:

2.1. If ε ∈ FIRST(α) then:

ε ∈ FIRST(Sα)

which contradicts rule (3) of the definition.

2.2. If ε ∉ FIRST(α) then:

    FIRST(α) ⊆ FIRST(S) (because β =>* ε)

=>  FIRST(α) ⊆ FIRST(Sα) ........ (I)

we also know that:

FIRST(α) ⊆ FOLLOW(S) ........ (II)

by (I) and (II), we have:

FIRST(Sα) ∩ FOLLOW(S) ≠ Ø

and since β =>* ε`, this contradicts rule (2) of the definition.

In every case we arrive at a contradiction, hence, a left-recursive grammar is not LL(1). ∎

Proposition 3: An ambiguous grammar is not LL(1).

Proof:

While the above proofs are mine, this one is not, it's by Kevin A. Naudé which I got from his answer that is linked below:

https://stackoverflow.com/a/18969767/6275103