# How can non-terminating $\lambda$-terms be turned into fixed-point combinators?

I've been thinking about these questions:

Is there a typed lambda calculus which is consistent and Turing complete?

https://cs.stackexchange.com/questions/65003/if-%CE%BB-x-x-x-has-a-type-then-is-the-type-system-inconsistent

and there are already some hard to answer related questions in the untyped setting! More specifically, I'm curious to know whether we can recover Turing-completeness from non-termination in the following way:

Question: Given a (pure) $\lambda$-term $t$ with no weak head normal form, does there always exist a fixed-point combinator $Y_t$ such that $$Y_t\ (\lambda x.x) = t$$

Equalities are all taken modulo $\beta\eta$.

I actually suspect this version of the question to be false, so one can relax the question to looping combinators, where a looping combinator $Y$ is defined to be a term such that for every $f$ $$Y\ f=f\ (Y'\ f)$$ where $Y'$ is again required to be a looping combinator. This is enough to define recursive functions as usual, of course.

More generally, I'm interested in finding "natural" ways to go from a non-terminating $t$ to a looping combinator, even if the above equation isn't satisfied.

I'm also interested in weaker versions of the above question, e.g. $t$ may be taken to be an application $t\equiv t_1\ t_2\ldots t_n$ with each $t_i$ in normal form (though I'm not sure that really helps).

So far: the natural approach is to take $t$ and "pepper" applications of $f$ throughout, e.g.

$$\Omega:=(\lambda x.x\ x)(\lambda x.x\ x)$$

becomes the usual

$$Y_\Omega:= \lambda f.(\lambda x. f\ (x\ x))\ (\lambda x.f\ (x\ x))$$

The idea is to reduce the head of $t$ to a lambda application $\lambda x. t'$ and replace it with $\lambda x.f\ t'$, but the next step is unclear (and I'm skeptical that this can lead to anything).

I'm not sure I understand enough about Böhm trees to see whether they have anything to say, but I highly doubt it, since $\Omega$'s Böhm tree is simply $\bot$, which looks nothing like the one for $Y_\Omega$: an infinite tree of abstractions.

Edit: A friend of mine remarked that this naive approach does not work with the term: $$(\lambda x.x\ x\ x)(\lambda x.x\ x\ x)$$ The naive approach would give $$(\lambda x.f\ (x\ x\ x))(\lambda x.f\ (x\ x\ x))$$ But this is not a fixed point combinator! This can be fixed by replacing the second application of $f$ by $\lambda y z.f\ y$, but then $f\mapsto \mathrm{I}$ does not give the original term. It's not clear whether this term is a counter-example to the original question though (and it certainly is not a counter example to the more general one).

• I believe the requirement that t has no head normal form should be strengthened to also exclude weak head normal forms. If t is able to produce a lambda, then, since in head position you always have a fixpoint combinator (starting with f=id), the lambda should be produced by it, that is not possible. Mar 15, 2017 at 16:25
• @AndreaAsperti you're correct, of course. I'll amend the question.
– cody
Mar 15, 2017 at 18:14

There are several aspects to this very nice question, so I will structure this answer accordingly. $\newcommand{\setof}[1]{\{#1\}}$ $\newcommand{\thra}{\twoheadrightarrow}$ $\newcommand{\codeof}[1]{\lceil #1 \rceil}$

1. The answer to the boxed question is no. The term $\Omega_3 = (\lambda x.xxx)(\lambda x.xxx)$ suggested by your friend is indeed a counterexample.

It was earlier noticed in the comments that one has counterexamples like the "ogre" $K^\infty=Y K$, until the question is restricted to terms without weak head normal form. Such terms are known as zero terms. These are terms which never reduce to a lambda, under any substitution.

For any fixed point combinator (fpc) $Y$, $Y I$ is a so-called mute (AKA "root-active") term: every reduct of it reduces further to a redex.

$K^\infty$ is not mute; neither is $\Omega_3$ $-$ as is manifest by inspecting its set of reducts, which is $$\setof{\Omega_3 \underbrace{(\lambda x. xxx) \cdots (\lambda x. xxx)}_k \mid k \in \mathbb{N}}$$

Rather than give a precise argument why $Y I$ is mute for all fpcs $Y$ (indeed, for any looping combinator) $-$ which may be laborious yet hopefully clear enough $-$ I will treat the obvious generalization of your question, restricting to mute terms as well.

Mute terms are a subclass of zero terms which are a subclass of unsolvable terms. Together these are perhaps the most popular choices for the concept of "meaningless" or "undefined" in the lambda calculus, corresponding to the trivial Berarducci, Levy-Longo, and B\"ohm trees, respectively. The lattice of notions of meaningless terms has been analyzed in detail by Paula Severi and Fer-Jan de Vries. [1] The mute terms constitute the bottom element in this lattice, i.e., the most restrictive notion of "undefined".

2. Let $M$ be a mute term, and $Y$ be a looping combinator with the property that $YI = M$.

First we argue that, for a fresh variable $z$, $Yz$ actually looks a lot like the $Y_M$ you described, obtained by "sprinkling $z$ around" some reduct of $M$.

By Church-Rosser, $YI$ and $M$ have a common reduct, $M'$. Take a standard reduction $R : YI \thra_s M'$. Every subterm of $M'$ corresponds to a unique subterm of $YI\equiv Yz[z:=I]$ under this reduction. For any subterm $C[N]=M'$, $R$ factors as $YI \thra C[N_0] \thra_{wh} C[N_1] \thra_i C[N]$, where the middle leg is a weak head reduction (and final leg is internal). $N$ is "guarded" by a $z$ iff this second leg contracts some redex $I P$, with $I$ a descendant of the substitution $[z:=I]$.

Obviously, $Y$ has to guard some subterms of $M$, for otherwise it would be mute as well. On the other hand, it must be careful not to guard those subterms which are needed for non-termination, for otherwise it could not develop the infinite B\"ohm tree of a looping combinator.

It thus suffices to find a mute term in which every subterm, of every reduct, is needed for non-normalization, in the sense that putting a variable in front of that subterm yields a normalizing term.

Consider $\Psi = W W$, where $W = \lambda w. w I w w$. This is like $\Omega$, but at every iteration, we check that the occurrence of $W$ in the argument position is not "blocked" by a head variable, by feeding it an identity. Putting a $z$ in front of any subterm will eventually yield a normal form of shape $zP_1\cdots P_k$, where each $P_i$ is either $I$, $W$ or a "$z$-sprinkling" of these. So $\Psi$ is a counterexample to the generalized question.

THEOREM. There is no looping combinator $Y$ such that $YI = \Psi$.

PROOF. The set of all reducts of $\Psi$ is $\setof{WW,WIWW,IIIIWW,IIIWW,IIWW,IWW}$. In order to be convertible with $\Psi$, $YI$ must reduce to one of these. The argument is identical in all cases; for concreteness, suppose that $YI \thra IIWW$.

Any standard reduction $YI \thra_s IIWW$ can be factored as \begin{align*} YI \thra_w P N_4, P \thra_w Q N_3, Q \thra_w N_1 N_2, \text{thus } YI \thra_w N_1 N_2 N_3 N_4\\ N_1 \thra I, N_2 \thra I, N_3 \thra W, N_4 \thra W \end{align*}

Let us refer to the reduction $YI \thra_w N_1 N_2 N_3 N_4$ as $R_0$, and the reductions starting from $N_i$ as $R_i$.

These reductions can be lifted over the substitution $[z:=I]$ to yield \begin{align*} R^z_0 : Yz \thra z^k(M_1 M_2 M_3 M_4)\\ N_i \equiv M_i[z:=I] \end{align*} so that $R_0$ is the composition $YI \stackrel{R^z_0[z:=I]}{\thra} I^k(N_1 \cdots N_4) \thra^k_w N_1 \cdots N_4$.

Similarly, we can lift each $R_i : N_i \thra N \in \setof{I,W}$ as \begin{align*} R^z_i : M_i \thra N^z_i\\ R_i : N_i \stackrel{R^z_i[z:=I]}{\thra} N^z_i[z:=I] \thra_I N \end{align*}

The second leg of this factorization of $R_i$ consists precisely of contracting those $I$-redexes which are created by the substitution $N^z_i[z:=I]$. (In particular, since $N$ is a normal form, so is $N^z_i$.)

$N^z_i$ is what we called a "$z$-sprinkling of $N$", obtained by placing any number of $z$s around any number of subterms of $N$. Since $N \in \setof{I,W}$, the shape of $N^z_i$ will be one of

\begin{align*} &z^{k_1}(\lambda x. z^{k_2}(x))\\ &z^{k_1}(\lambda w. z^{k_2}( z^{k_3}( z^{k_5}( z^{k_7}(w) z^{k_8}(\lambda x. z^{k_9}(x)) ) z^{k_6}(w) ) z^{k_4}(w) )) \end{align*}

So $M_1 M_2 M_3 M_4 \thra N^z_1 N^z_2 N^z_3 N^z_4$, with $N^z_i$ a $z$-sprinkling of $I$ for $i =1,2$ and of $W$ for $i=3,4$.

At the same time, the term $N^z_1 N^z_2 N^z_3 N^z_4$ should yet reduce to yield the infinite fpc Bohm tree $z(z(z(\cdots)))$. So there must exist a "sprinkle" $z^{k_j}$ in one of the $N^z_i$ which comes infinitely often to the head of the term, yet does not block further reductions of it.

And now we are done. By inspecting each $N^z_i$, for $i \le 4$, and each possible value of $k_j$, for $j \le 2+7\lfloor \frac{i-1}{2} \rfloor$, we find that no such sprinkling exists.

For example, if we modify the last $W$ in $IIWW$ as $W^z = \lambda w. z(wIww)$, then we get the normalizing reduction $$IIWW^z \to I W W^z \to WW^z \to W^z I W^z W^z \to z (I I I I) W^z W^z \thra z I W^z W^z$$

(Notice that $\Omega$ admits such a sprinkling precisely because a certain subterm of it can be "guarded" without affecting non-normalization. The variable comes in head position, but enough redexes remain below.)

3. The "sprinkling transformation" has other uses. For example, by placing $z$ in front of every redex in $M$, we obtain a term $N = \lambda z. M_z$ which is a normal form, yet satisfies the equation $N I = M$. This was used by Statman in [2], for example.

4. Alternatively, if you relax the requirement that $Y I = M$, you can find various (weak) fpcs $Y$ which simulate the reduction of $M$, while outputting a chain of $z$s along the way. I am not sure this would answer your general question, but there are certainly a number of (computable) transformations $M \mapsto Y_M$ which output looping combinators for every mute $M$, in such a way that the reduction graph of $Y_M$ is structurally similar to that of $M$. For example, one can write $$Y \codeof{M} z = \begin{cases} z (Y \codeof{P[x:=Q]} z) &M\equiv (\lambda x.P)Q\\ Y \codeof{N} z &M \text{ is not a redex and }M \to_{wh} N \end{cases}$$

[1] Severi P., de Vries FJ. (2011) Decomposing the Lattice of Meaningless Sets in the Infinitary Lambda Calculus. In: Beklemishev L.D., de Queiroz R. (eds) Logic, Language, Information and Computation. WoLLIC 2011. Lecture Notes in Computer Science, vol 6642.

[2] Richard Statman. There is no hyperrecurrent S,K combinator. Research Report 91–133, Department of Mathematics, Carnegie Mellon University, Pittsburgh, PA, 1991.

• This answer is great, and I will likely accept it. However, I'm not sure what the actual theorems you are describing, other than "there is no looping combinator $Y$ such that $Y\ I=\Omega_3$". I think stating the theorems separately will make the arguments much easier to follow.
– cody
Mar 19, 2017 at 16:27
• Good point. I just updated the answer. Mar 19, 2017 at 22:39