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- Subject: Re: case statement for Lua
- From: Mark@... (Mark Ian Barlow)
- Date: Mon, 14 Apr 1997 12:06:12 GMT
In message <199704131744.NAA03710@flatcoat.cs.virginia.edu>
Norman Ramsey writes:
> I felt the need for a case statement in Lua, so I added one.
> I borrowed syntax from Modula-3 for the case statement (with one
> slight change); here's an example:
> x = "bar"
> case x of
> | "foo" => y = 1
> | "bar" => y = 2
> else => y = 99
... and then there was some discussion of the aesthetics. I don't really
like the rather cryptic tokens '|' and '=>' either; they're too terse and
mathematical-looking for a language that is supposed to be friendly to
"casual" users, as Lua is (and all extension languages *should* be). Also,
at present, all control tokens are english words and all infix operators
are (!isalnum()); it would be a shame to break this consistency.
Looking at the above, (and the patch to lua.stx) I see:
SwitchIntroToken <expr> ExprTerminalToken
<case> ::= CaseIntroToken <value> ValTerminalToken <statement> |
But as Norman says, it would be much better if the statements were replaced
with blocks of statements. If this were done, I think it would be much more
consistent with the rest of Lua if we had:
<case> ::= <value> ValTerminalToken <block> BlockTerminalToken |
DefaultCaseToken <block> BlockTerminalToken
... and both ListTerminalToken and BlockTerminalToken were the good old "end"
used everywhere else. Norman has the following:
SwitchIntroToken ::= "case"
ExprTerminalToken ::= "of"
CaseIntoToken ::= "|"
ValTerminalToken ::= "=>"
DefaultCaseToken ::= "else"
ListTerminalToken ::= "end"
But I would suggest something like:
SwitchIntroToken ::= "with"
ExprTerminalToken ::= "choose"
ValTerminalToken ::= "does"
DefaultCaseToken ::= "default"
BlockTerminalToken ::= "end"
ListTerminalToken ::= "end"
Yielding a translated and expanded example:
with x choose
y = 1 z = "A" end
y = 99 z = "" end
y = 2 z = "B" end
Notice the arbitrary placement of the default case; "else" looks like it
*has* to go at the end but "default" looks OK anywhere. (Read on to see
why I don't think we need to force it to be last.)
I'm not too sure about "with x choose", however, and I'd welcome a better
I take Norman's point in his later post about explicit unique tokens
delimiting the segments of all control constructs, and omitting
CaseIntroToken ('|' in Norman's implementation) seems to break this,
but the inside of a case list is a special environment, and I have a
As implemented Norman's case construct is not a lot more efficient than
a long "if elseif... else end", although the introduction of DUP helps.
In my original post in response to LHF's "wishlist" call, asking for a case
construct I envisaged it being used when you had, say, >10 choices to cover
and being implemented by using the value of the control expression to index a
hidden table of anonymous functions, each of which would take no parameters
and return nothing.
This would give a real efficiency gain when there are a lot of choices by
leveraging Lua's built-in hashing abilities (Is this not what case statements
are _for_?). The above syntax would, I think, be cleaner if this method were
used, and the "default" case can go anywhere; it is just another table entry
like the rest, but with a special unique index...
-- Mark Ian Barlow Non-Linear Control Consultants Ltd.
Mark@nlcc.demon.co.uk Voice / Fax: +44 (0)1207 562 154