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From: | Elias Mårtenson |
Subject: | Re: [Bug-apl] Bug in the parser? |
Date: | Wed, 26 Nov 2014 23:38:22 +0800 |
Hi again,
I have analyzed this a bit further. Below are my conclusions.
1. Problem description
--------------------------------
Elias' initial question can be reduced to this: Given APL values A and B, say
A←1 2 3 4 5 and
B←1 1 0 1 0
how shall the following expressions be evaluated:
A /⍨ B
A (/⍨) B
(A) /⍨ B
The matter is a bit complicated by the ambiguity of the / token:
For historical reasons the token / is ambiguous and could mean the dyadic function compress or
the monadic operator reduce. GNU APL resolves this ambiguity at ⎕FX time when possible. For example,
in 1 2 3 / B the operator / is "downgraded" from "operator reduce" to "function compress" because 1 2 3 is a value
and therefore only function compress makes sense. However in A / B it is not known at ⎕FX time whether A is a function
or a value. In that case a symbol is assumed to be a function (and hence / an operator) while something in parentheses like
(A) is assumed to be a value. That is the reason why A /⍨ B gives a syntax error while (A) /⍨ B does not. IBM APL2 also
downgrades / from operator reduce to function compress, but at a later time.
The behavior of IBM APL2 in that special case is somewhat sub-optimal because insisting in / being an operator even
if it is obviously not leads to the following inconsistency:
IBM APL2:
1 (+¨) 1
2
1 (/¨) 1
SYNTAX ERROR
GNU APL:
1 (+¨) 1
2
1 (/¨)1
1
2. Alternatives
---------------------
I have tested what happens if we would introduce a M M pattern into GNU APL in order to
get IBM APL2's behavior. In the above examples (I used ¨ instead of Elias' original ⍨ because ¨ is
present in IBM APL2 while ⍨ is not). (+¨) is reduced by a pattern F M (function monadic-operator) to a
derived function. In contrast (/¨) is not reduced because there is no M M pattern (except in the cases where
/ was downgraded). The M M is shifted rather than reduced and the first F in a sequence F M M ... causes
the whole chain to be unrolled from left to right, This is the difference that Jay has observed between IBM APL2
and the others.
Adding a M M rule forces the parser to reduce M M immediately rather than shifting it. After doing that, a few regression
testcases did fail. Looking at the test results my impression was that the current behavior of GNU APL is the preferred one.
3. Conclusion
--------------------
My conclusion so far is that we should leave things as they are.
Putting things in parentheses is, in my opinion, not such a bad thing and it makes programs more explicit and more portable.
When choosing between:
A (/⍨) B and
(A) /⍨ B
I would recommend the former because that expresses better what is desired and the latter may change at some point in time.
/// Jürgen
On 11/25/2014 04:01 PM, Juergen Sauermann wrote:
Hi Jay,
yes, what I meant is that / is called like a dyadic function as in 1 1 1 / 1 2 3.
But handling it always like an operator could be a better solution.
Currently in GNU APL operators are distinguished from functions which works well
except for / and friends which are parsed as function in some contexts and parsed as operator in others.
I will look into changing this to making operators accept a non-function left argument.
/// Jürgen
On 11/25/2014 03:33 PM, Jay Foad wrote:
On 25 November 2014 at 14:06, Jay Foad <address@hidden> wrote:On 25 November 2014 at 13:38, Juergen Sauermann <address@hidden> wrote:I have read the IBM binding rules a number of times but they seem not to help. The problem of these rules is that they give different results in the cases where / is an operator and where / is a function.In IBM APL2 / is always an operator.For example: 1 2/¨3 4 ⍝ GNU APL, NARS2000 and Dyalog: parse as 1 2(/¨)3 4 3 4 4 1 2/¨3 4 ⍝ APL2: parse as (1 2/)¨3 4 3 3 3 4 4 4 Jay.
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