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Mini-Tamsin

This is just the first few parts of the spec ("Micro-Tamsin" plus variables and things) that the Tamsin compiler written in Tamsin can handle.

-> Tests for functionality "Intepret Tamsin program"

Fundaments

A Tamsin program consists of one or more productions. A production consists of a name and a parsing rule (or just "rule" for short). Among other things, a rule may be a non-terminal, which is the name of a production, or a terminal, which is a literal string in double quotes. (A full grammar for Tamsin can be found in Appendix A.)

When run, a Tamsin program processes its input. It starts at the production named main, and evaluates its rule. A non-terminal in a rule "calls" the production of that name in the program. A terminal in a a rule expects a token identical to it to be on the input. If that expectation is met, it evaluates to that token. If not, it raises an error. The final result of evaluating a Tamsin program is sent to its output.

(If it makes it easier to think about, consider "its input" to mean "stdin", and "token" to mean "character"; so the terminal "x" is a command that either reads the character x from stdin and returns it (whence it is printed to stdout by the main program), or errors out if it read something else. Or, thinking about it from the other angle, we have here the rudiments for defining a grammar for parsing a trivial language.)

| main = blerf.
| blerf = "p".
+ p
= p

| main = blerf.
| blerf = "p".
+ k
? expected 'p' found 'k'

Productions can be written that don't look at the input. A rule may also consist of the keyword return, followed a term; this expression simply evaluates to that term and returns it. (More on terms later; for now, think of them as strings.)

So, the following program always outputs blerp, no matter what the input is.

| main = return blerp.
+ fadda wadda badda kadda nadda sadda hey
= blerp

Note that in the following, blerp refers to the production named "blerp" in one place, and in the other place, it refers to the term blerp. Tamsin sees the difference because of the context; return must be followed by a term, while a parsing rule cannot be part of a term.

| main = blerp.
| blerp = return blerp.
+ foo
+ foo
+ foo 0 0 0 0 0
= blerp

A rule may also consist of the keyword print followed by a term, which, when evaluated, sends the term to the output, and evaluates to the term. (Mostly this is useful for debugging. In the following, world is repeated because it is both printed, and the result of the evaluation.)

| main = print hello & print world.
+ ahoshoshohspohdphs
= hello
= world
= world

A rule may also consist of two subrules joined by the & operator. The & operator processes the left-hand side rule. If the LHS fails, then the & expression fails; otherwise, it continues and processes the right-hand side rule. If the RHS fails, the & expression fails; otherwise it evaluates to what the RHS evaluated to.

| main = "a" & "p".
+ ap
= p

| main = "a" & "p".
+ ak
? expected 'p' found 'k'

| main = "a" & "p".
+ ep
? expected 'a' found 'e'

If you are too used to C or Javascript or the shell, you may use && instead of &.

| main = "a" && "p".
+ ap
= p

A rule may also consist of two subrules joined by the | operator. The & operator processes the left-hand side rule. If the LHS succeeds, then the | expression evaluates to what the LHS evaluted to, and the RHS is ignored. But if the LHS fails, it processes the RHS; if the RHS fails, the | expression fails, but otherwise it evaluates to what the RHS evaluated to.

For example, this program accepts 0 or 1 but nothing else.

| main = "0" | "1".
+ 0
= 0

| main = "0" | "1".
+ 1
= 1

| main = "0" | "1".
+ 2
? expected '1' found '2'

If you are too used to C or Javascript or the shell, you may use || instead of |.

| main = "0" || "1".
+ 1
= 1

Using return described above, this program accepts 0 or 1 and evaluates to the opposite. (Note here also that & has a higher precedence than |.)

| main = "0" & return 1 | "1" & return 0.
+ 0
= 1

| main = "0" & return 1 | "1" & return 0.
+ 1
= 0

| main = "0" & return 1 | "1" & return 0.
+ 2
? expected '1' found '2'

Evaluation order can be altered by using parentheses, as per usual.

| main = "0" & ("0" | "1") & "1" & return ok.
+ 011
= ok

Note that if the LHS of | fails, the RHS is tried at the position of the stream that the LHS started on. This property is called "backtracking".

| ohone = "0" & "1".
| ohtwo = "0" & "2".
| main = ohone | ohtwo.
+ 02
= 2

Note that print and return never fail. Thus, code like the following is "useless":

| main = foo & print hi | return useless.
| foo = return bar | print useless.
= hi
= hi

Note that return does not exit the production immediately — although this behaviour may be re-considered...

| main = return hello & print not_useless.
= not_useless
= not_useless

Alternatives can select code to be executed, based on the input.

| main = aorb & print aorb | cord & print cord & return ok.
| aorb = "a" & print ay | "b" & print bee.
| cord = "c" & print see | eorf & print eorf.
| eorf = "e" & print ee | "f" & print eff.
+ e
= ee
= eorf
= cord
= ok

And that's the basics. With these tools, you can write simple recursive-descent parsers. For example, to consume nested parentheses containing a zero:

| main = parens & "." & return ok.
| parens = "(" & parens & ")" | "0".
+ 0.
= ok

| main = parens & "." & return ok.
| parens = "(" & parens & ")" | "0".
+ (((0))).
= ok

(the error message on this test case is a little weird; it's because of the backtracking. It tries to match (((0))) against the beginning of input, and fails, because the last ) is not present. So it tries to match 0 at the beginning instead, and fails that too.)

| main = parens & "." & return ok.
| parens = "(" & parens & ")" | "0".
+ (((0)).
? expected '0' found '('

(the error message on this one is much more reasonable...)

| main = parens & "." & return ok.
| parens = "(" & parens & ")" | "0".
+ ((0))).
? expected '.' found ')'

To consume a comma-seperated list of one or more bits:

| main = bit & {"," & bit} & ".".
| bit = "0" | "1".
+ 1.
= .

| main = bit & {"," & bit} & ".".
| bit = "0" | "1".
+ 0,1,1,0,1,1,1,1,0,0,0,0,1.
= .

(again, backtracking makes the error a little odd)

| main = bit & {"," & bit} & ".".
| bit = "0" | "1".
+ 0,,1,0.
? expected '.' found ','

| main = bit & {"," & bit} & ".".
| bit = "0" | "1".
+ 0,10,0.
? expected '.' found '0'

Comments

A Tamsin comment is introduced with # and continues until the end of the line.

| # welcome to my Tamsin program!
| main = # comments may appear anywhere in the syntax
|        # and a comment may be followed by a comment
|   "z".
+ z
= z

Variables

When a production is called, the result that it evaluates to may be stored in a variable. Variables are local to the production.

| main = blerp → B & blerp & "." & return B.
| blerp = "a" | "b".
+ ab.
= a

Note that you don't have to use the Unicode arrow. You can use an ASCII digraph instead.

| main = blerp -> B & blerp & "." & return B.
| blerp = "a" | "b".
+ ab.
= a

Names of Variables must be Capitalized.

| main = blerp → b & return b.
| blerp = "b".
?

In fact, the result of not just a production, but any rule, may be sent into a variable by →. Note that → has a higher precedence than &.

| main = ("0" | "1") → B & return B.
+ 0
= 0

A → expression evaluates to the result placed in the variable.

| main = ("0" | "1") → B.
+ 0
= 0

This isn't the only way to set a variable. You can also do so unconditionally with set.

| main = eee.
| eee = set E = whatever && set F = stuff && return E.
+ ignored
= whatever

And note that variables are subject to backtracking, too; if a variable is set while parsing something that failed, it is no longer set in the | alternative.

| main = set E = original &
|          (set E = changed && "0" && "1" | "0" && "2") &
|        return E.
+ 01
= changed

| main = set E = original &
|          (set E = changed && "0" && "1" | "0" && "2") &
|        return E.
+ 02
= original

Terms

We must now digress for a definition of Tamsin's basic data type, the term.

A term T is defined inductively as follows:

In fact, there is little theoretical difference between an atom and a constructor with zero subterms, but they are considered different things for conceptual clarity.

Note that EOF is not at atom.

A term is called ground if it does not contain any variables.

Terms support an operation called expansion, which also requires a context C (a map from variable names to ground terms.)

The result of expansion will always be a ground term.

Ground terms support an operation called flattening (also sometimes called stringification).

The result of flattening is always an atom.

Ground terms also support an operation called reprifying (also sometimes called "readable stringification"). It is very similar to flattening, but results in an atom, the contents of which is always a legal syntactic atom in term context in a Tamsin program. (Flattening a term does not always guarantee this because, for example, flattening '\n' results in an actual newline.)

Note that in the above, "printable" means ASCII characters between 32 (space) and 126 ~. It is not dependent on locale.

Also, \xXX escapes will always be output in lowercase, e.g. \x0a, not \x0A.

The input to a Tamsin production is, in fact, an atom (although it's hardly atomic; "atom" is sort of a quaint moniker for the role these objects play.)

The contexts in Tamsin which expect a term expression are return, set, and arguments to productions (but you haven't seen those yet.) In these contexts, a bareword evaluates to an atom (rather than a non-terminal.)

| main = return hello.
= hello

But an atom can contain arbitrary text. To write an atom which contains spaces or other things which are not "bareword", enclose it in single quotes.

| main = return Hello, world!
? expected

| main = return 'Hello, world!'.
= Hello, world!

Note that the atom 'X' is not the same as the variable X. Nor is the atom 'tree(a,b)' the same as the constructor tree(a,b).

In a term context, a constuctor may be given with parentheses after the string.

| main = return hello(world).
= hello(world)

The bareword rule applies in subterms.

| main = return hello(beautiful world).
? expected

| main = return hello('beautiful world').
= hello(beautiful world)

In a term context, variables may be given. The term is always expanded during evaluation.

| main = set E = world & return hello(E).
= hello(world)

A term expression may also contain a + operator, which evaluates and flattens both its arguments and concatenates the resulting atoms.

| main = set E = world & return 'hello, ' + E + '!'.
= hello, world!

And note, underscores are allowed in production and variable names, and atoms without quotes.

| main = this_prod.
| this_prod = set Var_name = this_atom & return Var_name.
= this_atom

Escape Sequences

A literal string may contain escape sequences. Note, I hate escape sequences! So I might not leave this feature in, or, at least, not quite like this.

| main = "a" & "\"" & "b" & print 'don\'t'.
+ a"b
= don't
= don't

| main = "a" & "\\" & "b" & print 'don\\t'.
+ a\b
= don\t
= don\t

| main = "a" & "\n" & "b" & print 'don\nt'.
+ a
+ b
= don
= t
= don
= t

| main = "a" & "\t" & "b" & print 'don\tt'.
+ a b
= don   t
= don   t

The escape sequence \x must be followed by two hex digits.

# | main = "a" & "\x4a" & "b" & print 'don\x4at'.
# + aJb
# = donJt
# = donJt

Note also that you can print a constructor.

| main = print hi(there('I\'m'(a(constructor)))).
= hi(there(I'm(a(constructor))))
= hi(there(I'm(a(constructor))))

Examples using Terms

This program accepts a pair of bits and evaluates to a term, a constructor pair, with the two bits as subterms.

| main = bit → A & bit → B & return pair(A, B).
| bit = "0" | "1".
+ 10
= pair(1, 0)

| main = bit → A & bit → B & return pair(A, B).
| bit = "0" | "1".
+ 01
= pair(0, 1)

This program expects an infinite number of 0's. It will be disappointed.

| main = zeroes.
| zeroes = "0" & zeroes.
+ 00000
? expected '0' found 'EOF'

This program expects a finite number of 0's, and returns a term representing how many it found. It will not be disappointed.

| main = zeroes.
| zeroes = ("0" & zeroes → E & return zero(E)) | return nil.
+ 0000
= zero(zero(zero(zero(nil))))

We can also use concatenation to construct the resulting term as an atom.

| main = zeroes.
| zeroes = ("0" & zeroes → E & return E + 'Z') | return ''.
+ 0000
= ZZZZ

Implicit set and return

Unquoted atoms and constructors ("barewords") can have the same names as productions. If they are used in rule context, they are assumed to refer to productions. If they are used in term context, they are assumed to refer to terms.

| main = blerf.
| blerf = return blerf.
= blerf

Because variable names cannot be mistaken for productions, if they are used in rule context and followed by ←, this is equivalent to set.

| main = S ← blerf & "x" & return S.
+ x
= blerf

There is of course an ASCII digraph for the left-pointing arrow. (The right-pointing symbol in the input in this test is just to get keep my text editor's syntax highlighting under control.)

| main = S <- blerf & "x" & return S.
+ x->
= blerf

If the variable name is not followed by ←, this is an implied return of the variable's value.

| main = S ← blerf & "x" & S.
+ x
= blerf

If a quoted term (atom or constructor) is used in rule context, this too cannot be mistaken for a production. So this, too, implies a return of that term.

| main = S ← blerf & "x" & 'frelb'.
+ x
= frelb

(Not so sure about this one. It makes the grammar compflicated.)

# | main = S ← blerf & "x" & 'frelb'(S).
# + x
# = frelb(blerf)

But it must be quoted, or Tamsin'll think it's a production.

| main = S ← blerf & "x" & frelb.
+ x
?

Aside: ← vs. →

One may well ask why Tamsin has both →, to send the result of a rule into a variable, and ←, to send a term into a variable, when both of these could be done with one symbol, in one direction, and in fact most languages do it this way (with a symbol like =, usually.)

Two reasons:

This way gives us two disjoint syntax contexts (rule context and term context) which lets us re-use the same symbols (such as lowercased barewords) for dual purposes. Which in turn lets us write more compact code.

And also, parsing is a linear process. When we consume tokens from the input, whether directly with a terminal, or indirectly via a non-terminal, we want them to be easily located. We want all our ducks to be in a row, so to speak. This setup ensures that the focus of parsing is always on the left and not nested inside a term. Like so:

| main = "(" &
|        expr → S &
|        "," &
|        expr → T &
|        U ← pair(S,T) &
|        ")" &
|        U.
| expr = "a"
|      | "b"
|      | "c".
+ (b,c)
= pair(b, c)

That said, it is possible to use only the → if you like, by using return (or implicit return!) instead of set. Like so:

| main = "(" &
|        expr → S &
|        "," &
|        expr → T &
|        return pair(S,T) → U &
|        ")" &
|        U.
| expr = "a"
|      | "b"
|      | "c".
+ (b,c)
= pair(b, c)

In my opinion, this style is not as clear, because at the rule which updates U, U itself is the focus and should be on the left.

What about the other way around? We could introduce some symbol (say, /) which allows a rule in what would otherwise be a term context, for example

main = "(" &
       S ← /expr &
       "," &
       T ← /expr &
       U ← pair(S,T) &
       ")" &
       U.
expr = "a"
     | "b"
     | "c".

This would also work, and is more similar to conventional programming languages; however, in my opinion, it is not as clear either, because in the rules which parse the sub-expressions, it is expr that is the focus of the logic, rather than the variables the results are being sent into.

Static Checking

Note that the production named by a non-terminal must exist in the program, even if it is never evaluated.

| main = "k" | something_undefined.
+ k
? something_undefined

Advanced Parsing

eof

If there is more input available than what we wrote the program to consume, the program still succeeds.

| main = "a" & "p".
+ apparently
= p

The built-in production eof may be used to match against the end of the input (colloquially called "EOF".)

| main = "a" & "p" & eof.
+ ap
= EOF

This is how you can make it error out if there is extra input remaining.

| main = "a" & "p" & eof.
+ apt
? expected EOF found 't'

The end of the input is a virtual infinite stream of EOF's. You can match against them until the cows come home. The cows never come home.

| main = "a" & "p" & eof & eof & eof.
+ ap
= EOF

any

The built-in production any matches any token defined by the scanner except for EOF. (Remember that for now "token defined by the scanner" means "character", but that that can be changed, as you'll see below.)

| main = any & any & any.
+ (@)
= )

| main = any & any.
+ a
? expected any token, found EOF

Optional rules

The rule [FOO] is a short form for (FOO | return nil).

| main = ["0"].
+ 0
= 0

| main = ["0"].
+ 
= nil

So we can rewrite the "zeroes" example to be simpler:

| main = zeroes.
| zeroes = ["0" & zeroes → E & return zero(E)].
+ 0000
= zero(zero(zero(zero(nil))))

Iterated rules

The rule {FOO} is what it is in EBNF, and/or a while loop. Like [], we don't strictly need it, because we could just write it as recursive BNF. But it's handy. Like while loops are handy. It returns the result of the last successful rule applied, or nil if none were successful.

| main = {"0"}.
+ 0 0 0 0
= 0

| main = {"0"}.
+ 1 2 3 4
= nil

Backtracking applies to {} too.

| zeroesone = {"0"} & "1".
| zeroestwo = {"0"} & "2".
| main = zeroesone | zeroestwo.
+ 000002
= 2

So we can rewrite the "zeroes" example to be even... I hesistate to use the word "simpler", but we can... write it differently.

| main = zeroes.
| zeroes = set Z = nil & {"0" && set Z = zero(Z)} & return Z.
+ 0000
= zero(zero(zero(zero(nil))))

fail

The built-in production fail always fails. This lets you establish global flags, of a sort. It takes a term, which it uses as the failure message.

| debug = return ok.
| main = (debug & return walla | "0").
+ 0
= walla

| debug = fail notdebugging.
| main = (debug & return walla | "0").
+ 0
= 0

| main = set E = 'Goodbye, world!' & fail E.
+ hsihdsihdsih
? Goodbye, world!

!

The ! ("not") keyword is followed by a rule. If the rule succeeds, the ! expression fails. If the rule fails, the ! expression succeeds. In neither case is input consumed — anything the rule matched, is backtracked. Thus ! should almost always be followed by & and something which consumes input, such as any.

| main = !"k" & any.
+ l
= l

| main = !"k" & any.
+ k
? expected anything except

| main = !("k" | "r") & any.
+ l
= l

| main = !("k" | "r") & any.
+ k
? expected anything except

| main = !("k" | "r") & any.
+ r
? expected anything except

This is particularly useful for parsing strings and comments and anything that contains arbitrary text terminated by a sentinel.

| main = "'" & T ← '' & {!"'" & any → S & T ← T + S} & "'" & return T.
+ 'any bloody
+   gobbledegook *!^*(^@)(@* (*@#(*^*(^(!^
+ you like.'
= any bloody
=   gobbledegook *!^*(^@)(@* (*@#(*^*(^(!^
= you like.

Dynamic Terminals

As mentioned, the terminal "foo" matches a literal token foo in the buffer. But what if you want to match something dynamic, something you have in a variable? You can do that with «»:

| main = set E = f & «E».
+ f
= f

| main = set E = f & «E».
+ b
? expected 'f' found 'b'

Note that you don't have to use the Latin-1 guillemets. You can use the ASCII digraphs instead.

| main = set E = f & <<E>>.
+ b
? expected 'f' found 'b'

Terms are flattened for use in «». So in fact, the "foo" syntax is just syntactic sugar for «'foo'».

| main = «'f'».
+ f
= f

Oh, and since we were speaking of sentinels earlier...

| main = {sentineled → A & print A & {" "}} & return ok.
| sentineled =
|    "(" &
|    any → S &
|    T ← '' & {!«S» & any → A & T ← T + A} & «S» &
|    ")" &
|    T.
+ (!do let's ))) put &c. in this string!)   (&and!this!one&)
= do let's ))) put &c. in this string
= and!this!one
= ok

folds

The following idiom is essentially a fold from functional programming.

| main = T ← '' & {$:alnum → S & T ← T + S} & return T.
+ dogwood
= dogwood

It is so common, that Tamsin supports a special form for it. The infix operator / takes a rule on the left-hand side, and a term (used as the initial value) on the right-hand side, and expands to the above.

| main = $:alnum/''.
+ dogwood
= dogwood

| main = $:alnum/'prefix'.
+ dogwood.
= prefixdogwood

You can use any rule you desire, not just a non-terminal, on the LHS of /.

| main = ("0" | "1")/'%'.
+ 0110110110.
= %0110110110

Note that the RHS of / is a term expression, so it can contain a +.

| main = ("0" | "1")/'%' + '&'.
+ 0110110110.
= %&0110110110

If there is an additional /, it must be followed by an atom. This atom will be used as a constructor, instead of the concat operation.

| main = $:alnum/nil/cons.
+ dog.
= cons(g, cons(o, cons(d, nil)))

Note that the middle of // is a term expression.

| main = $:alnum/cat+food/cons.
+ dog.
= cons(g, cons(o, cons(d, catfood)))

Note that the RHS of // is not a term expression.

| main = $:alnum/ni+l/co+ns.
+ dog.
? expected

Not that (for now) /'s cannot be nested. But you can make a sub-production for this purpose.

| main = ("*" & string)/nil/cons.
| string = $:alnum/''.
+ *hi*there*nice*day*isnt*it
= cons(it, cons(isnt, cons(day, cons(nice, cons(there, cons(hi, nil))))))

System Module

The module $ contains a number of built-in productions which would not be possible or practical to implement in Tamsin. See Appendix C for a list.

In fact, we have been using the $ module already! But our usage of it has been hidden under some syntactic sugar.

| main = $:expect(k).
+ k
= k

| main = $:expect(k).
+ l
? expected 'k' found 'l'

The section about aliases needs to be written too.

Here's $:alnum, which only consumes tokens where the first character is alphanumeric.

| main = "(" & {$:alnum → A} & ")" & A.
+ (abc123deefghi459876jklmnopqRSTUVXYZ0)
= 0

| main = "(" & {$:alnum → A} & ")" & A.
+ (abc123deefghi459876!jklmnopqRSTUVXYZ0)
? expected ')' found '!'

Here's $:upper, which only consumes tokens where the first character is uppercase alphabetic.

| main = "(" & {$:upper → A} & ")" & A.
+ (ABCDEFGHIJKLMNOPQRSTUVWXYZ)
= Z

| main = "(" & {$:upper → A} & ")" & A.
+ (ABCDEFGHIJKLMNoPQRSTUVWXYZ)
? expected ')' found 'o'

Here's $:startswith, which only consumes tokens which start with the given term. (For a single-character scanner this isn't very impressive.)

| main = "(" & {$:startswith('A') → A} & ")" & A.
+ (AAAA)
= A

| main = "(" & {$:startswith('A') → A} & ")" & A.
+ (AAAABAAA)
? expected ')' found 'B'

Here's $:mkterm, which takes an atom and a list and creates a constructor.

| main = $:mkterm(atom, list(a, list(b, list(c, nil)))).
= atom(a, b, c)

Here's $:unquote, which takes three terms, X, L and R, where L and R must be one-character atoms. If X begins with L and ends with R then the contents in-between will be returned as an atom. Otherwise fails.

| main = $:unquote('"hello"', '"', '"').
= hello

| main = $:unquote('(hello)', '(', ')').
= hello

| main = $:unquote('(hello)', '(', '"').
? term '(hello)' is not quoted with '(' and '"'

Here's $:equal, which takes two terms, L and R. If L and R are equal, succeeds and returns that term which they both are. Otherwise fails.

Two atoms are equal if their texts are identical.

| main = $:equal('hi', 'hi').
= hi

| main = $:equal('hi', 'lo').
? term 'hi' does not equal 'lo'

Two constructors are equal if their texts are identical, they have the same number of subterms, and all of their corresponding subterms are equal.

| main = $:equal(hi(there), hi(there)).
= hi(there)

| main = $:equal(hi(there), lo(there)).
? term 'hi(there)' does not equal 'lo(there)'

| main = $:equal(hi(there), hi(here)).
? term 'hi(there)' does not equal 'hi(here)'

| main = $:equal(hi(there), hi(there, there)).
? term 'hi(there)' does not equal 'hi(there, there)'

Here's $:emit, which takes an atom and outputs it. Unlike print, which is meant for debugging, $:emit does not append a newline, and is 8-bit-clean.

| main = $:emit('`') & $:emit('wo') & ''.
= `wo

-> Tests for functionality "Intepret Tamsin program (pre- & post-processed)"

$:emit is 8-bit-clean: if the atom contains unprintable characters, $:emit does not try to make them readable by UTF-8 or any other encoding. (print may or may not do this, depending on the implementation.)

# | main = $:emit('\x00\x01\x02\xfd\xfe\xff') & ''.
# = 000102fdfeff0a

-> Tests for functionality "Intepret Tamsin program"

Here's $:repr, which takes a term and results in an atom which is the result of reprifying that term (see section on Terms, above.)

| main = $:repr(hello).
= hello

| main = $:repr('016fo_oZZ').
= 016fo_oZZ

| main = $:repr('016fo$oZZ').
= '016fo$oZZ'

| main = $:repr('').
= ''

| main = $:repr('016\n016').
= '016\x0a016'

| main = $:repr(hello(there, world)).
= hello(there, world)

| main = V ← '♡' & $:repr('□'(there, V)).
= '\xe2\x96\xa1'(there, '\xe2\x99\xa1')

| main = $:repr(a(b(c('qu\'are\\')))).
= a(b(c('qu\'are\\')))

# | main = $:repr('\x99').
# = '\x99'

Here's $:reverse, which takes a term E, and a term of the form X(a, X(b, ... X(z, E)) ... ), and returns a term of the form X(z, X(y, ... X(a, E)) ... ). The constructor tag X is often cons or pair or list and E is often nil.

| main = $:reverse(list(a, list(b, list(c, nil))), nil).
= list(c, list(b, list(a, nil)))

E need not be an atom.

| main = $:reverse(list(a, list(b, list(c, hello(world)))), hello(world)).
= list(c, list(b, list(a, hello(world))))

If the tail of the list isn't E, an error occurs.

| main = $:reverse(list(a, list(b, list(c, hello(world)))), nil).
? malformed list

If some list constructor doesn't have two children, an error occurs.

| main = $:reverse(list(a, list(b, list(nil))), nil).
? malformed list

The constructor tag can be anything.

| main = $:reverse(foo(a, foo(b, foo(c, nil))), nil).
= foo(c, foo(b, foo(a, nil)))

But if there is a different constructor somewhere in the list, well,

| main = $:reverse(foo(a, fooz(b, foo(c, nil))), nil).
? malformed list

You can reverse an empty list.

| main = $:reverse(nil, nil).
= nil

But of course,

| main = $:reverse(nil, zilch).
? malformed list

This is a shallow reverse. Embedded lists are not reversed.

| main = $:reverse(list(a, list(list(1, list(2, nil)), list(c, nil))), nil).
= list(c, list(list(1, list(2, nil)), list(a, nil)))

Here's gensym.

| main = $:gensym('foo').
= foo1

| main = $:gensym('foo') → F & $:gensym('foo') → G & $:equal(F, G).
? 'foo1' does not equal 'foo2'