What is the story behind 'goto' vs 'go' 'to' in ALGOL-60?

Question

Preamble

As Algol-60 had no reserved words, the language keywords had to be specially marked, e.g. prefixed with an underscore with a trailing space or semicolon (_begin _integer i;i:=42_end), or enclosed in quotes ('begin''real'pi;pi:=3.14'end'), or made distinct from regular identifiers in some other way. Note that in both cases all unnecessary spaces are deliberately omitted in those two examples.

I have two Algol-60 compilers working. One uses quotes for keywords and considers them as atomic tokens.

The other, dated 1st of May 1979, uses the underscore as a "mode switch" character up to the next non-alphabetical symbol, so one underscore at the beginning of a keyword is enough, but extra underscores don't hurt:

   1.   _BE__GIN_ _I_N_T_EGER_ I;
   2.  I:=42; OUTPUT(‘Z’, I) _EN_D

―――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――

               42.000000

That being said, it follows that writing _BEGIN_INTEGER will not fly: an error "В ЯЗЫКЕ НЕТ СЛОВА BEGININTEGER", translated as "THERE IS NO WORD BEGININTEGER IN THE LANGUAGE" will follow.

Note that an underscore before a space character does not hurt either. However, any space in the middle of a keyword will result in the pieces considered as separate keywords:

   1.   _BE _GIN _END

     СТРОКА 1
   В ЯЗЫКЕ НЕТ СЛОВА    BE

     СТРОКА 1
   В ЯЗЫКЕ НЕТ СЛОВА    GIN

GOTOs

Now, to the point. I had a vague recollection that Algol-60 allowed two ways to write the GOTO operator. Indeed, according to Algol 60 syntax versions, the Revised Report (1963) prescribes goto, whereas the Modified Report (1976) prescribes go to.

The compiler which uses quotes, dated 25th of January 1979, only allows 'GOTO'. Nothing to see there, moving along.

The other behaves as follows:

_BEGIN _GOTO L; L:_END naturally, works, as does

   1.   _BEGIN ____G_O_____T_O____ L; L:_END

as well as various other combinations of extra underscores; but

   1.   _BEGIN _GO
   2.  _TO L; L:_END

works just as well, to satisfy both variants of the syntax.

For a final experiment, what would an error message be for _BEGIN _GO _FOR L; L:_END? Somewhat unexpectedly, the compiler produces В ЯЗЫКЕ НЕТ СЛОВА GO ("there is no word GO in the language").

Question

If there is absolutely no semantic difference between the two spellings of the GOTO operator, why bother implementing both? Or, to start with, why bother introducing ambiguity in the standard, forcing the compiler writers to waste valuable machine words to support somebody's pet peeve? (Those two were rhetorical, do not attempt to flag the question "opinion-based".)

I'm hoping there is a captivating story of heated discussions related to the space in "goto"/"go to" somewhere in the memoirs of the authors of the language, or their interviews. Is there?

PS. The Electrologica X1 compiler by Dijkstra/Zonneveld (1960) uses an "optimistic" parsing mechanism: begin goto a; a: end (which could be shortened to be g a; a: en) produces the expected

023640   1 0000000140  START
023641   1 0522023642    GOTO 023642 A
023642   1 0000000141  STOP

whereas begin go to a; a: end results in a bizarre

023640   1 0000000140  START
023641   1 0522023643    GOTO 023643 A
023642   1 0000000075  MUL
023643   1 0000000141  STOP

with a stray multiplication opcode, which happens to be absolutely harmless, as it would never be executed. Thus it appears that the first Algol-60 compiler happened to accept both variants of the "go to" statement by chance.

Answer 1

Here's a different take on the issue (thus a separate answer).

There is no such distinction.

I ran this program through the KDF9 Whetstone Algol translator:

_b_e_g_i_n_i_n_t_e_g_e_rn; n:=1;
_e_n_d

It compiles just fine(though of course there's always the possibility of artifacts from the modern ASCII translation). My interpretation is as follows:

Spaces have no significance per language fiat, though their appearance within basic symbols is not explicitly mentioned. Because there is a small and finite set of delimiters in Algol60, a recognizer can trivially know that begininteger is the two tokens begin and integer (here I use boldface since I don't know how to underline in this markup).

That being the case, _g_o_t_o (or goto) is not distinguishable from _g_o _t_o (or go to).

You can regard underlining as a single-character shift prefix. With that interpretation, the lexer knows when it has recognized a basic symbol by its content, not by a terminating mark.

This does not readily transfer to stropping regimes that use enclosing characters (like 'go to' or "go to"), and indeed the same compiler refuses 'begininteger'n (diagnostic: no begin). I suppose the obvious implementation here is to isolate the word by closing delimiter and then decide if it's a valid symbol.

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Postscriptum

I took a look at Randell & Russell's Algol 60 Implementation, which is the book on the Whetstone Algol implementation. They describe a sort of lexical-analysis buffer that's 12 symbols long (where a symbol seems to include an underline flag that has been added, and spaces have already been discarded). When an underlined character appears at the output end for compilation, an attempt is made to match a known basic symbol in the remainder of the buffer - i.e., as I suspected, we're checking against known symbols, not expecting any detectable separator character.

(If that sounds weird to modern readers, don't forget that we're compiling from paper tape, and since I/O time dominates, the goal is to be able to compile in between characters being read. So the compiler is running a few characters behind the input routine).

And significantly for this question:

Naturally, spaces, changes to a new line, spurious case definition characters(*), etc., are allowed between the underlined characters of any basic symbol.

(*) footnote from me: Flexowriter paper tape code included 'case normal' and 'case shift' characters. It was essentially a 6-bit code.

With respect to notations like 'begin', I'm now not sure whether they were even allowed in this particular compiler. They might be part of the online front-end, since underlining without a dead underline key is horrible. I'll poke around some more when I get a moment.

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Postpostscriptum

I'm now running Whetstone papertape Algol locally under ee9.

The sequence _g _o _t _o (boldface equivalent g o t o) is perfectly acceptable, demonstrating for sure that spaces are insignificant.

Apostrophe stropping -- thus 'goto' -- is not accepted by the compiler.

Kidsgrove Algol does accept apostrophe stropping, but will only take 'goto', not 'go to' or 'go' 'to'.

tl;dr results mixed.

Answer 2

One may call it an issue of taste or linguistics. The point is that there isn't a English word 'goto'. These are two words. In fact, 'goto' would be pronounced different from 'go to'.

Hold your breath and take a step back. Try to view it and shed 40+ years of FORTRAN and BASIC indoctrination. Try to pronounce it like someone uninitiated with programming. It sounds awful, doesn't it? Way off the meaning. It almost hurts. ALGOL was, in its abbreviated way, meant to be readable by humans (*1).

The way later Sinclair ZX80 (4 KiB) BASIC is a great example of the same issue. The machine was meant to appeal to non professional users. The key words GOTO and GOSUB were spelled out as GO TO and GO SUB (*2).

Of course this was easy due the special way the Sinclairs used to enter keywords. When building a character input-based compiler, it gets a lot more effort to detect two 'words' as part of a single keyword. So obviously writing GOTO helps a lot ... except it sucks when reading.

This was a real war in the 1960s. Pick your side.

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*1 - Heck, isn't that one of the reasons HLLs were developed in the first place?

*2 - It got changed with the ZX81, to be more in line with 'regular' machines. Pressure of the 'experts' I guess.

Answer 3

(Opinion - I doubt there is any answer that is not opinion, unless one of the parties involved shows up here).

I think it is just personal taste.

Algol Bulletin 38 contains a letter (page 8) signed by the future authors of the Modified Report, and there the separate go to is used.

Section 4.3, page 18, bears the heading Go to statements.

This apparently passed unremarked in the bulletin. so perhaps it was not regarded as controversial.

Contrariwise, AB39.3.2, page 32 contains a letter from Wichmann, one of the authors of the letter cited previously, which writes goto.

So it's difficult to conclude that there was any particular intent one way or the other.