acl: Generalized Combinatory Logic Interpreter

version 1.2

last revision: 2011-06-12

Bruce Ediger

Table of contents

  1. Introduction
  2. Starting
  3. Using
  4. Defining Primitives
  5. Expressing Bracket Abstraction Algorithms
  6. Interpreter Commands
  7. Examples
  9. Building and Installing


This document describes how to build and use acl v1.2.

acl ("Any Combinatory Logic") interprets programming languages with similarities to various "Combinatory Logic" (CL) formal systems. It doesn't interpret any "Combinatory Logic" in that it runs on computers with finite CPU speed and a finite memory. Most or all formal systems fail to take these limits into account.

acl differs from other interpreters in that the user must specify any primitives to the interpreter. It has no built-in atomic primitive combinators.

acl does not have any built-in bracket abstraction algorithms. The user must specify a bracket abstraction algorithm before doing bracket abstraction on a term. The user must specify primitives before using them in a bracket abstraction rule definition.

Without specifying any primitives, the interpreter checks the syntax of "applicative structures", where every atomic term constitutes a free variable.

Release 1.2 of acl includes '*^' special symbol in bracket abstraction specifications.

Another page documents the design and implementation of a less general Combinatory Logic interpreter. That document still holds as a good description of the interpretation of applicative structures that acl performs.

Starting the interpreter

After downloading or building the interpreter's executable, you can start it from the command line:

 7:57AM 87 % ./acl

The interpreter uses "ACL>" as its prompt for user input. acl has a strict grammar, so you must type in either a term for reduction, or an interpreter command, or a command to examine a term.

A keyboard interrupt (almost always control-C) can interrupt whatever long-running reduction currently takes place. A keyboard interrupt at the "ACL>" prompt will cause the interpreter to exit.

You have to use keyboard end-of-file (usually control-D) to exit acl.

Giving the interpreter a CL term causes it to execute the usual functional language interpreter's read-eval-print loop. The interpreter prints the input in a minimally-parenthesized representation, reduces to normal form, and prints a text representation of the normal form. It prints a prompt, and waits for another user input.

acl does standard Combinatory Logic "normal order" evaluation. The leftmost outermost contraction gets evaluated first.

Command line options

-cenable reduction cycle detection
-ddebug contractions
-eelaborate output
-L filenameInterpret a file named filename before reading user input
-N numberperform up to number contractions on each input expression.
-pDon't print any prompt.
-ssingle-step reductions
-T numberevaluate an expression for up to number seconds
-ttrace reductions

The -e or -s options have no use without the -t option, but -t alone might have some use.

-L filename can occur more than one time. acl will interpret the files in the order they appear on the command line. After interpreting the last (or only) file, it prints the ACL> prompt, then waits for interactive user input. This command line flag pre-loads files. To interpret files during an interactive session, use the load command.

Using the interpreter

Interactive input

I designed acl for use as an interactive system, with a user typing CL expressions at a prompt. The interpreter reduces the expression to a normal form (if it has one), or hits some other limit, like a pre-set timeout, count of allowed contractions or the user's patience.

After entering an entire expression, the user types "return" or "enter" to trigger evaluation.

The built-in prompt for input is the string "ACL>". It appears when the interpreter starts up, or has finished reducing whatever expression the user gave it last, or it has executed an interpreter command.

You have to type an end-of-file character (almost always control-D) to quit, as it has no built-in "exit" or "quit" command.

A keyboard interrupt (almost always control-C) can interrupt whatever long-running reduction currently takes place, returning the user to the ACL> prompt. A keyboard interrupt at the "ACL>" prompt will cause the interpreter to exit.

Non-interactive input

The -p command line option causes the interpreter to not print a prompt. This only has use for non-interactive input. The interpreter does read from stdin and write to stdout. You can use it as a non-interactive "filter", with input and output redirection.

Grammar, briefly

Expressions consist of either a single term, or two (perhaps implicitly parenthesized) terms. Terms consist of either a user-defined primitive or a variable, or a parenthesized expression. The reduce command, and bracket abstraction each produce an expression.

Variables (which can also serve as abbreviations or names of user-defined primitives) look like C or Java style identifiers. An identifier consists of a letter, followed by zero or more letters or underscores. Variables, abbreviations and user-defined primitives share the same name space. You can't have an abbreviation with the same name as a primitive.

The interpreter treats combinators and variables as "left associative", the standard in the Combinatory Logic literature. That means that an expression like this: I a b c d ends up getting treated as though it had parentheses like this: ((((I a) b) c) d)

To apply one complex term to another, the user must parenthesize terms. Applying W (W K) to C W would look like this: (W (W K)) (C W).


Users can parenthesize input expressions as much or as little as they desire, up to the limits of left-association and the meaning they wish to convey to the interpreter. The grammar used by acl does not allow single terms inside paired parentheses. It considers strings like "(I)" as syntax errors. You have to put at least two terms inside a pair of parentheses. Parentheses must have a match.

The interpreter prints out normal forms in minimal-parentheses style. Users have the opportunity to cut-n-paste output back into the input, as output has valid syntax. No keyboard shortcuts exist to take advantage of any previous output.

Defining Primitives

acl does not implement any built-in primitives. The user must describe desired primitives to the interpreter.

Rules specifying primitives

Defining a primitive means getting the interpreter to read a line having this form:

rule: Name ni ... ->  mi ...

The Name becomes the primitive's name when used in a CL expression. Name has the same format as a C or Java language identifier: a letter followed by zero or more letters, digits or underscores.

The ni symbols represent ascending-value digits, beginning with 1. These consitute the required arguments of the primitive under definition.

The mi symbols also represent digits, which must also appear as ni arguments. They represent the positions of arguments after the primitive reduces. You can delete, rearrange, duplicate and compose arguments, but you can't introduce other non-argument results. You can only define "proper" combinators. You can define "regular" or "irregular" combinators.


The interpreter imposes no limits on number of primitives the user defines, or on the number of arguments a given primitive uses or produces. Defining large numbers of primitives will slow it down, as will reducing a primitive that has a large number of arguments, or produces a large number of results in a reduced term.

You can redefine a primitive with another rule: input which uses the name to be redefined. You cannot delete a primitive, once you have defined it. You have to exit the interpreter to "delete" pimitives.

Primitives versus abbreviations

At first glance, a user defined primitive and an abbreviation appear redundant. Differences exist, both subtle and gross.

One gross difference: primitives only rearrange, delete, duplicate or regroup their arguments. An abbreviation (in the form of a bracket abstracted expression) can insert primitives. No way to construct an "improper" primitive exists, but abbreviations can do this. Abbreviations can refer to a fixed point combinator. You cannot define a fixed point combinator as a primitive.

acl abbreviations work in a way that encourages the user to build up complex terms by abbreviating smaller, simpler terms, then invoking all the abbreviations together. For example, you could build up a fixed-point combinator like this:

ACL> def subexpr (x y x)
ACL> def Y ([x,y] subexpr)([y,x] (y subexpr))

This contrived example illustrates the use of abbreviating a common sub-expression, as well as demonstrating that an abbreviation isn't "atomic". The bracket abstractions capture what the user defines as free variables.

A more subtle distinction exists in that primitives reduce atomically: normal-order evaluation doesn't take place "inside" a primitive. An example follows to clarify.

ACL> rule: S 1 2 3 -> 1 3 (2 3)
ACL> rule: I 1 -> 1
ACL> def M (S I I)
ACL> trace on     show expression after each contraction
ACL> detect on    mark reducible primitives with '*'
ACL> step on      single-step, pause after each contraction
S I I (S I I)
continue? return
[2] I* (S I I) (I* (S I I))   even out-of-order contractible primitives marked with '*'
continue? return
[2] S* I I (I* (S I I))
continue? return
[4] I* (I* (S I I)) (I* (I* (S I I)))
continue? return
[3] I* (S I I) (I* (I* (S I I)))
continue? return
[3] S* I I (I* (I* (S I I)))
continue? return
[6] I* (I* (I* (S I I))) (I* (I* (I* (S I I))))
continue? q return

Each abbreviation M gets expanded into a term S I I. The S I I terms get evaluated in normal order, leftmost-outermost contraction happens first. This leads to an infinitely expanding term, not the M MM M cycle that one might naively hope for.

To achieve an M MM M cycle, the user could define M as a primitive.

ACL> rule: M 1 -> 1 1
ACL> cycles on    detect cyclical reductions
ACL> trace on     show expression after each contraction
ACL> detect on    mark reducible primitives with '*'
[1] M* M
[1] M* M
Found a pure cycle of length 1, 1 terms evaluated, ends with ".M M"
[1] M* M

Expressing Bracket Abstraction Algorithms

"Bracket abstraction" names the process of creating a CL expression without specified variables, that when evaluated with appropriate arguments, ends up giving you the original expression with the specified variables.

The acl interpreter uses the conventional square-bracket notation. For example, to create an expression that will duplicate its single argument, one would type:

ACL> [x] x x

You can use more than one variable inside square brackets, separated with commas:

ACL> [a, b, c] a (b c)

The above square-bracketed expression ends up performing three bracket abstractions, abstracting c from a (b c), b from the resulting expression, and a from that expression.

A bracket abstraction makes an expression. You can use abstractions where you might use any other simple or complex expression, defining an abbreviation, a sub-expression of a much larger expression, as an expression to evaluate immediately, or inside another bracket abstraction. For example, you could create Turing's fixed-point combinator like this:

ACL> def U [x][y] (x(y y x))
ACL> def Yturing (U U)

Note the use of nested bracket abstractions. The abstraction of y occurs first, then x gets abstracted from the resulting expression.

You could express [x][y] (x(y y x)) with the alternate form [x,y] (x(y y x)). The same nested abstraction occurs.

acl allows you to express even complicated bracket abstraction algorithms. You can write rules that match specific terms, or that match general sub-expressions.

Bracket Abstraction Rules

You input the abstraction algorithm in the form of rules, one per line, in decreasing order of priority. Each line has this format:

abstraction: [_] lhs -> rhs

The lexical token "[_]" denotes the abstraction of whatever variable the user chooses. It must appear after the abstraction: token. It can also appear in the right-hand-side of an abstraction rule, where it will cause recursive abstraction(s).

Left Hand Side

The left-hand-side (lhs) looks like a valid CL term, except that it can contain one or more special symbols, as well as names of primitives and/or variables:

Right Hand Side

The right-hand-side (rhs) also looks like a valid CL term, except that it can contain one or more special symbols, as well as the names of primitives and/or variables:

Rule Precedence

The order in which the user enters rules amounts to specifying precedence. The first rule entered at the ACL> prompt has the highest precedence. The last rule entered has the lowest precedence.

The user must define primitives before using them in a right-hand-side. Otherwise, what appears as a "primitive" will actually constitute a free variable with a confusingly identical name.

Bracket Abstraction Rule Examples

The rule format above allows expression of the standard SKI-basis 4-rule algorithm like this:

  1. abstraction: [_] *- -> K 1
  2. abstraction: [_] _ -> I
  3. abstraction: [_] *- _ -> 1
  4. abstraction: [_] * * -> S ([_] 1) ([_] 2)

In rule 1, the *- on lhs gets into the resulting expression as the '1' on the right-hand-side.

Similarly, in rule 4, the two expressions on the lhs, each denoted by an '*', have the variable abstracted ('[_]') in the rhs.

The 9-rule bracket abstraction algorithm from John Tromp's Binary Lambda Calculus and Combinatory Logic looks like this:

  1. abstraction: [_] S K * -> S K
  2. abstraction: [_] *- -> K 1
  3. abstraction: [_] _ -> S K K
  4. abstraction: [_] *- _ -> 1
  5. abstraction: [_] _ * _ -> [_] S S K _ 2
  6. abstraction: [_] (*! (*! *)) -> [_] (S ([_]1) 2 3)
  7. abstraction: [_] (*! * *!) -> ([_] S 1 ([_]3) 2)
  8. abstraction: [_] *! *^ (*! *^) -> [_] (S 1 3 2)
  9. abstraction: [_] * * -> S ([_] 1) ([_] 2)

Tromp writes rule 5 above as:

λ2x.(x M x) ≡ λ2x.(S S K x M)

The abstracted variable x appears in the right- and left-hand-side of the rule as _ . Tromp's M appears as * on lhs and 2 on rhs of the acl abstraction rule.

The right-hand-side of the rule references the abstracted-out-variable. The digit 2 in the RHS refers to the * (match any term) marker in the LHS. The RHS also builds a term which has that variable re-abstracted.

Rules 7 and 9 above feature the '*!' special symbol, which means "term containing no variables whatsoever", a.k.a. a combinator. Tromp seeks to minimize the size of the end result of multi-variable abstractions. Variables exist only to get abstracted away, so a term containing multiple variables will undergo multiple abstractions. Applying rules 6 and 7 to terms more than once ends up making the resulting variable-free terms much larger than they otherwise would end up.

Rule 8 above features the '*^' special symbol. This rule triggers an abstraction when it finds two, lexically-identical sub-expressions, in the positions described. The two expressions in the RHS denoted by '*!' do not get checked for lexical equality, but must not contain variables for the rule to trigger an abstraction.

The "*^" special symbol can appear more than once, should you find it interesting to do so. All sub-trees marked with a '*^' must compare identically to trigger the abstraction rule. More than one abstraction rule can contain '*^' symbols, but the lexical identity only gets checked during examination of a single rule. Lexical identity does not get checked "across rules".

Interpreter Commands

Defining abbreviations

define and def allow a user to introduce "abbreviations" to the input later. Using define or def causes acl to keep an internal copy of the expression so abbreviated. When the abbreviation appears in an input expression, acl puts a copy of the internal-held expression in the input. No matter how complex the expression, an abbreviation comprises a single term. Effectively, the interpreter puts the expanded abbreviation inside a pair of parentheses.

def makes an easy-to-type abbreviation of define.

The reduce command actually produces an expression, just like a bracket abstraction. Unlike define or def, you can use reduce anywhere an expression would fit, as part of a larger expression, as part of an abbreviation, or as part of a bracket abstraction.

reduce causes an out-of-order (normal order, leftmost outermost) reduction to take place. The expression next to reduce gets interpreted, and the result put back into the original context. reduce allows a user to store the normal form of an expression, rather than just a literal expression, as def and define don't cause any contractions to take place.

Information about expressions

print lets you see what abbreviations expand to, without evaluation, as does printc. The "=" sign lets you determine lexical equality. All combinators, variables and parentheses have to match as strings, otherwise "=" deems the expressions not equivalent. You can put in explicit reduce commands on both sides of an "=", otherwise, no evaluation takes place.

size and length seem redundant, but authorities measure CL expressions different ways. These two methods should cover the vast majority of cases.

Canonical Expression Representation

The printc command, and cycle detection output use a canonical form of representing a CL expression.

In this case, "canonical" means: pre-order, depth-first, left-to-right traversal of the parse tree, output of a period (".") for each application, and a space before each combinator or variable.

A simple application (K I, for example) looks like this: .K I

A more complex application (P R (Q R)) looks like this: ..P R.Q R

The advantage to this sort of notation is that every application appears explicitly, and variant, semantically-equivalent parentheses don't appear.

Detecting reduction cycles

Some CL expressions end up creating a cycle. M M or W W W constitute examples from common bases. After a certain number of contractions, the interpreter encounters an expression it has previously created. If you issue the cycles on command, the interpreter keeps track of every expression in the current reduction, and stops when it detects a cyclical reduction.

detect on causes the interpreter to count and mark possible contraction (with an asterisk), regardless of reduction order. It does "normal order" reduction, but ignores that for the contraction count. This has use with trace on.

Turning cycle detection on will add time to an expression's reduction, as will possible contraction detection.

Intermediate output and single-stepping

You can issue any of these commands without an "on" or "off" argument to inquire about the current state of the directive.

trace, debug and elaborate provide increasingly verbose output. trace shows the expression after each contraction, debug adds information about which branch of an application it evaluates, and elaborate adds to that information useful in debugging memory allocation problems.

detect causes trace to also print a count of possible contractions (not all of them normal order reductions), and mark contractable primitives with an asterisk.

step on causes the interpreter to stop after each contraction, print the intermediate expression, and wait, at a ? prompt for user input. Hitting return goes to the next contraction, n or q terminates the reduction, and c causes it to quit single-stepping.

trace on will also display steps taken during bracket abstractions.

Reduction information and control

You can turn time outs off by using a 0 (zero) second timeout. Similarly, you can turn contraction-count-limited evaluation off with a 0 (zero) count.

timer on also times bracket abstraction.

Reading in files

You have to double-quote filenames with whitespace or non-alphanumeric characters in them. You can use absolute filenames (beginning with "/") or you can use filenames relative to the current working directory of the acl process.

Printing primitive and abstraction rules

Rules about primitives and abstraction rules should appear in a format appropriate for cut-n-paste, that is, in input syntax. Abstraction rules should appear in order of precedence, highest precedence first.


The following examples demonstrate either an interesting facet of Combinatory Logic (Klein fourgroup, Grzegorzyk bracket abstraction, AMEN basis) or illustrates part of the interpreter that I found hard to describe (abstraction rules in Tromp's bracket abstraction, and Scott Numerals example). Your tastes may vary.


Building and installing

I developed this program on Slackware Linux 12.0, and Arch Linux. I used the C compilers GCC, LCC, PCC, TCC and Clang. I tried to make it as ANSI-C as possible by using various compilers, not allowing any compiler warnings when building, and using as many warning flags for GCC compiles as possible.

I expect it will build on any *BSD-based system, but I haven't done that formally. For a BSD system, try make cc.


Licensed under GNU Public License v2, or later.


  1. Get the source code. You can:
  2. Unpack source code. From the command line: tar xzf acl-1.2.tar.gz
  3. Change directory. From the command line: cd acl-1.2
  4. Compile source code. From the command line: make cc uses traditionally-unix-named tools, and may work better on Solaris or the BSDs. Traditional, AT&T derived lex will require a minor edit to file lex.l. You can see what to uncomment at the very top of the file.
  5. At this point, you can test the newly-compiled executable. From the command line: ./runtests. Most of the tests should run quite rapidly, in under a second. At least two of the tests run for 30 seconds or so, and at least one of the tests provokes a syntax error message from the interpreter.
  6. Install the interpreter wherever you want, or you can execute it in-place. To install, use the cp or mv commands to move or copy the executable to where ever you want it. It does not care what directory it resides in, and it does not look for configuration files anywhere.