Cody Boone Ferguson
22e483723c
Instead of code blocks the bug / (mis)features statuses are quoted rather than in a code block. In the case that there is more than one a line break was added at the end of all but the last one. A bug status was renamed to be slightly more correct (maybe more than one was renamed for the same purpose but at least one was). The bug statuses in the quotes are now bold to let them stand out better.
336 lines
11 KiB
Markdown
336 lines
11 KiB
Markdown
## To build:
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``` <!---sh-->
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make
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```
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### Bugs and (Mis)features:
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The current status of this entry is:
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> **STATUS: INABIAF - please DO NOT fix**
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For more detailed information see [2019/lynn in bugs.html](../../bugs.html#2019_lynn).
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## To use:
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``` <!---sh-->
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./prog
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```
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## Try:
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``` <!---sh-->
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./try.sh
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```
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## Judges' remarks:
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A fully functional compiler. The example prints out the 30th [Fibonacci
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number](https://en.wikipedia.org/wiki/Fibonacci_sequence).
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## Author's remarks:
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### Remarks:
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A Haskell compiler. Supports a subset of Haskell more than large enough to
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self-host. Like GHC with custom language extensions:
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* `WeDontServeYourType`: Compilation failing because of inscrutable type
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checking rules? Confused by the ever-growing mire of extensions to the type
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system? The solution is simple: no type checking. It also means no
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typeclasses. Just pass dictionaries explicitly.
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* `ZeroPauseGarbageCollection`: Instead of disruptive stop-the-world garbage
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collection, we only tidy up when the world stops on its own accord, that is,
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on program termination.
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* `OneHundredPercentPure`: Gone is the catch-all lawless IO monad. And no
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trace of those scary `unsafeThisAndThat` functions. All functions must be pure.
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* `SyntaxForTheMasses`: See below.
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### Building:
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Build the compiler:
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``` <!---sh-->
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cc -o prog prog.c
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```
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### Demos:
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#### Fibonacci numbers:
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Test the compiler on [fib.hs](%%REPO_URL%%/2019/lynn/fib.hs):
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``` <!---sh-->
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(./prog < fib.hs ; cat prog.c) > fib.c
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```
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Compiling the output produces a binary that prints the 30th Fibonacci number.
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The file [ghcfib.hs](%%REPO_URL%%/2019/lynn/ghcfib.hs)
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includes [fib.hs](%%REPO_URL%%/2019/lynn/fib.hs) with some glue code,
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and shows GHC also accepts our subset of Haskell:
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``` <!---sh-->
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ghc ghcfib.hs
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```
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#### Self-hosting compiler:
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To avoid revealing the original Haskell source, we
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instead provide [hint.hs](%%REPO_URL%%/2019/lynn/hint.hs), the output of a certain stage of the
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compiler when run on itself. This intermediate output is hopefully difficult to
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understand, yet is accepted by our compiler:
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### NOTICE to those who wish for a greater challenge:
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**If you want a greater challenge, don't read any further**:
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just try to understand the program without running the command below.
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If you get stuck, come back and run the following command:
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``` <!---sh-->
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(./prog < hint.hs ; cat prog.c) > hint.c
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make hint
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./hint
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```
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The output program behaves like the compiler itself.
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Unlike the original source from which it is derived, GHC fails to compile
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[hint.hs](%%REPO_URL%%/2019/lynn/hint.hs). This is because values have been replaced with their [Scott
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encodings](https://en.wikipedia.org/wiki/Mogensen-Scott_encoding)
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by this stage, which messes up type-checks; we'd need `equirecursive` types for
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it to work.
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#### Regexes:
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The file [lol.hs](%%REPO_URL%%/2019/lynn/lol.hs) contains an adaptation of [Doug McIlroy's elegant code from
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"Enumerating the strings of regular
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languages"](https://www.cs.dartmouth.edu/~doug/nfa.pdf). We exercise it by
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showing the first entries of the length-ordered list of all strings consisting
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of the characters `a` and `b` that contain an even number of `a`s.
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``` <!---sh-->
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(./prog < lol.hs ; cat prog.c) > lol.c
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make lol
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./lol
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```
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A GHC wrapper is provided:
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``` <!---sh-->
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ghc ghclol.hs
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```
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### Strongly-connected components:
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See [scc.hs](%%REPO_URL%%/2019/lynn/scc.hs), and its GHC wrapper [ghcscc.hs](%%REPO_URL%%/2019/lynn/ghcscc.hs),
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for an elegant way to print the strongly-connected components of a graph in reverse topological
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order.
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``` <!---sh-->
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(./prog < scc.hs ; cat prog.c) > scc.c
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```
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It expects the input to be in a similar format as a previous entry
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(2018 vokes). For example:
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``` <!---sh-->
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make scc
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./scc < example-1.txt
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./scc < example-2.txt
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```
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The output should agree, though our program omits line numbers and does not
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sort entries within a line. (Also, our program only treats spaces as
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whitespace, and supports any non-space character in a vertex name.)
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### Syntax
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Some claim Haskell syntax is frightening because braces and semicolons are
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optional. Some complain about a zoo of twisty little operators, all alike. Our
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compiler addresses such concerns by making braces and semicolons compulsory,
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disabling layout-based rules, making every operator left-associative with same
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precedence, and only defining 6 primitive functions:
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```
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: + - * / <=
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```
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The arithmetic operations behave as they do in C on `unsigned int`s, and `(:)` is
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Haskell's `cons` function. Like Haskell and unlike C, `(<=)` returns a `Bool`
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(which are Church booleans behind the scenes) and not an `Int`.
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No ifs, ands, or buts. Define them yourself. No tuples. Who needs them when
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algebraic data types delivers a better product? There is no do. No where. No
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list comprehensions. No unary operators.
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Let expressions and operator sections are supported.
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Within global scope or a let expression, each definition can only refer to
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itself or previous definitions. This implies we can only achieve mutual
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recursion by having one function pass itself to others.
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### Caveats
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The only primitive type is `Int`. In particular, they represent characters. To
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interoperate with GHC, our compiler treats any undefined functions as the
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`identity` function, so that we may freely use `ord` and `chr` (or `fromEnum` and
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`toEnum`) to make our code acceptable to both compilers.
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The main function is the last function to successfully parse. It should have
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type `[Int] -> [Int]`, and our compiler treats this like the function passed
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into `Prelude.interact`, that is, the entire standard input is passed to this
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function, and the result is printed to standard output.
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`Bool` must be defined as:
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```
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data Bool = True | False;
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```
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so that it matches the Scott-encoded booleans internally used by the primitive
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function `(<=)`.
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The alternatives in a `case` expressions must list every data constructor in
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the order they are defined. For example, if we have:
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```
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data Foo a b = Bar a | Baz | Qux Int [b]
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```
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then a `case` expression that examines a term of this type must have the form:
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```
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case x of
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{ Bar a -> ...
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; Baz -> ...
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; Qux n bs -> ...
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}
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```
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We stress braces and semicolons are required. Our fussy parser treats
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semicolons as separators, not terminators.
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The effect of erroneous input is undefined. It may be best to develop with GHC,
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but even then, be mindful of changes needed because of issues caused by the
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uniform operator precedence and the touchy format of `case` alternatives.
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### Why?
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One-letter variable names abound in IOCCC entries, and for good reason. These
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tiny pieces of confetti are hard to read, and leave room for more code. Then
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why not go further and use zero-letter variable names? That is, tacit
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programming or point-free style.
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I had been playing with an algorithm devised by Oleg Kiselyov that effortlessly
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and efficiently eliminates those pesky variables, leaving behind terms composed
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from a small set of combinators. No need for lambda lifting or supercombinators.
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By adding a handful of lines of mostly parsing code, we get a Haskell compiler,
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or rather, a compiler that accepts a subset of Haskell sufficiently large to
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self-host. You might say I wrote a tool for this contest, then ran it on itself
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to make an entry for it.
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### Obfuscation techniques:
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Even with Kiselyov's algorithm and some term rewriting, the compiler only fit
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after compression, which naturally obfuscates the code. More tricks were needed
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to fit within the size limits.
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* One-letter variables, until enough is done to banish variables completely.
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* Ipse dixit. Near the end, the code declares itself to be an obfuscated
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program. (There is also an exhortation intended for the judges in a similar
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format earlier in the source.)
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* Typical C mischief: pre- and post-increment, commutative array indexing,
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ternary operators, and so on.
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* Huffman coding.
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* Base-85 because high bits are frowned upon.
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* Mixed radix encoding to game iocccsize. From a past entry,
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[2018/bellard](../../2018/bellard/index.html), it seems 9 11 12 32 are the only
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whitespace octets that may appear verbatim in string literals.
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* Choosing what to encode in Huffman/base-85 and what to encode in mixed radix
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was a delicate balancing act. In the end, I only had a few bytes to spare, which
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I spent on gratuitous confusion.
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* The effects of some functions depend on the order their arguments are
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evaluated, yet the program works either way. Why?
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* Ugly macros for the runtime system's jump table for lazy reduction. A previous
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entry, [2013/endoh1](../../2013/endoh1/index.html), has a cuter solution, which I
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avoid because of originality concerns and also because my combinators are
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compressed.
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* Relies on the inability of C comments to nest.
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* Cheaper and more complicated to print comma-separated `int`s (and header and
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footer) in C.
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* Primitive functions use a trick described in depth by [Naylor and Runciman,
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"The Reduceron reconfigured and
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re-evaluated"](https://www.cs.york.ac.uk/fp/reduceron/jfp-reduceron.pdf). We
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represent the integer `n` with a term equivalent to `Y(BT)n`; it works because
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`Y(BT)ne = e(Y(BT)n)`.
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* A sentinel in the heap often confused me, so ought to confuse others.
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Other obfuscation techniques are better appreciated after decoding the
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compiler. See [hint.hs](%%REPO_URL%%/2019/lynn/hint.hs).
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* Mercilessly point-free. Everything is a combinator.
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* Scott encoding. Everything is a combinator.
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* Sum types are oddly ordered and may even contain unused data constructors to
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reduce code duplication. (In C terms, `union`s may have extra fields, and they're
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ordered in such a way so we can reuse code to access certain fields.)
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* The `undefined` function compiles to the `(.)` function.
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* Replaced the only `(&&)` with `(||)` and `not` using De Morgan's law, which
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makes some comparisons less comprehensible.
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* [Kiselyov only published "λ to SKI, Semantically" last
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year](http://okmij.org/ftp/tagless-final/ski.pdf).
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* Recursion via the fixpoint (`Y`) combinator, which is represented by the
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variable `""` during one stage of compilation.
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* Involves theory that may be less familiar to C programmers: maps, folds,
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parser combinators, lambda calculus, bracket abstraction, denotational
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semantics, etc.
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### Warnings:
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On my system, it compiles cleanly with `-Wall` with older standards, e.g.
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`-std=c89`, but less cleanly if `-pedantic` is also supplied.
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Compiling the compiler output with `-Wall` triggers a warning about a
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strange-looking comment.
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### Behind-the-scenes commentary:
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[My website reveals how this compiler
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works](https://crypto.stanford.edu/~blynn/compiler/ioccc.html).
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<!--
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Copyright © 1984-2024 by Landon Curt Noll. All Rights Reserved.
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You are free to share and adapt this file under the terms of this license:
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Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)
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For more information, see:
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https://creativecommons.org/licenses/by-sa/4.0/
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-->
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