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From: Rob Pike <robpike@gmail.com>
Date: Fri, 2 Jul 2021 14:40:09 +1000
Message-ID: <CAKzdPgw4PMS1uMLT1gJuz=yPDJ4tfo_9MWkKRiixh57JGtVobQ@mail.gmail.com>
To: Steve Johnson <scj@yaccman.com>
Content-Type: multipart/mixed; boundary="000000000000a114d905c61c8def"
Subject: Re: [TUHS] [tuhs] Dennis Ritchie's couch
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<resending with the program as an attachment, as 100K is considered big
here, and no images hidden in reply. moderator, you can kill the prior
messages. computers are hard.>

To show you what I mean, here is a parser I wrote recently for a simple Go
expression evaluator, in Go. It has all Go's precedence levels and
operators. The one odd detail is that >= etc. can be combined, as in 1 >= 2
> 3, but that doesn't matter in the context of this program and is easily
avoided with a few more lines in cmpList.

I'm using a screen grab because GMail refuses to leave my indentation
alone. I left factor off. It's got all the usual details but it's the leaf
of the grammar and of no interest here.

Note that this could easily have been made a table instead of a bunch of
one-line functions.

Parsers are easy to write. It took us a generation (or more) to understand
that, but it's remarkable nonetheless. The first big step might have been
realizing that recursion was a good idea, even if you weren't writing LISP,
if the data structure is itself recursive.

-rob

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<div dir=3D"ltr">&lt;resending with the program as an attachment, as 100K i=
s considered big here, and no images hidden in reply. moderator, you can ki=
ll the prior messages. computers are hard.&gt;<span class=3D"gmail-im" styl=
e=3D"color:rgb(80,0,80)"><div><br></div><div>To show you what I mean, here =
is a parser I wrote recently for a simple Go expression evaluator,=C2=A0in =
Go. It has all Go&#39;s precedence levels and operators. The one odd detail=
 is that &gt;=3D etc. can be combined, as in 1 &gt;=3D 2 &gt; 3, but that d=
oesn&#39;t matter in the context of this program and is easily avoided with=
 a few more lines in cmpList.<div><br></div><div>I&#39;m using a screen gra=
b because GMail refuses to leave my indentation alone. I left factor off. I=
t&#39;s got all the usual details but it&#39;s the leaf of the grammar and =
of no interest here.</div><div><br></div><div>Note that this could easily h=
ave been made a table instead of a bunch of one-line functions.</div><div><=
br></div><div>Parsers are easy to write. It took us a generation (or more) =
to understand that, but it&#39;s remarkable nonetheless. The first big step=
 might have been realizing that recursion was a good idea, even if you were=
n&#39;t writing LISP, if the data structure is itself recursive.</div><div>=
<br></div><div>-rob</div></div></span></div>

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