Tcl Improvement Proposals: Check-in [0b004d697b]

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Overview

Comment:	New TIP: 526
Downloads:	Tarball \| ZIP archive
Timelines:	family \| ancestors \| descendants \| both \| trunk
Files:	files \| file ages \| folders
SHA3-256:	0b004d697b9b97127aa6c3918e5c1e474f22b19ec499e81343d086f79af2de0d
User & Date:	dkf 2018-11-08 10:16:11.988

Context

2018-11-10
09:26		Add precisions in TIP #466 about spaces trimming, -spacemode options, and the fact that no -spacemode value shows text completely identical to the legacy code (slight backwards incompatibility). check-in: 489ef3ca2b user: fvogel tags: trunk
2018-11-08
10:16		New TIP: 526 check-in: 0b004d697b user: dkf tags: trunk
2018-11-06
15:18		Corrected vote record check-in: af2513fa1b user: dkf tags: trunk

Changes

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Added tip/526.md.
















































































































1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112	+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +	# TIP 526: Make [expr] Only Accept One Argument Author: Donal K. Fellows <[email protected]> State: Draft Type: Project Vote: Pending Created: 08-Nov-2018 Tcl-Version: 9.0 Post-History: ------ # Abstract This TIP proposes that `expr` should only accept a single argument and stop accepting and concatenating arguments. # Rationale and Discussion A very common fault of `expr` usage is to fail to brace expressions. This was semi-good practice in Tcl 7.6 and before as Tcl's substitution phase was faster than the expression evaluation engine's, but it was always a little tricky even then as it depended on you being sure that the substituted values were numeric for safety. When that critical side condition didn't hold, it resulted in unsafe code: proc addFive {x} { expr $x + 5 } puts [addFive 17]; # ==> 22 puts [addFive {[exec format C:]}]; # OOPS! From Tcl 8.0 onwards, this was no longer a good idea. The bytecode compiler is able to generate compiled code ahead of time for braced expressions, but is not able to do so (usefullly) if any of the arguments to `expr` are unbraced. Of course, we want to be able to use an unbraced argument occasionally, such as when we take an expression as an argument from elsewhere, but those are very much the unusual case: almost always, a call to `expr` with any argument unbraced is _both_ a performance bug and a security bug. Because this is so strongly the case, there are uses of Tcl that are almost always a problem; for example: expr $x >= "inf"; # where $x is expected to be a string # Proposal To reduce the incidence of this, I propose that in Tcl 9.0 the `expr` command should only take a single argument. It most definitely isn't a total fix for this — a total fix isn't really possible without making the `expr` command special in the Tcl base language, and that goes against the general principle that _nothing_ is special there — but it will at least reduce the incidence a bit by encouraging people to either brace their expressions or be very clear that they are not doing so. This will leave virtually all correct code working, and yet force code that has a problem to acknowledge the fact. Supported cases: 1. Braced expressions: expr {$x * 5} 2. Expressions in variables: set theExpression {$x * 5} expr $theExpression 3. Expressions from commands: expr [dict get $myExpressions thisCase] Cases that work because we can't stop them without breaking the language: 1. Expressions in double quotes: expr "$x * 5" 2. Expressions with unquoted compound substitutions: expr $x5+[llength $y] Cases that are outright rejected: 1. Expressions with multiple arguments: expr $x 5 2. Expressions with multiple literal arguments (uncommon): expr {$x} 5 ## Workarounds for Broken Old Code The cases where someone genuinely needs the old behaviour can be handled by them using `concat` (or `string cat`, depending on exact requirements) explicitly to make the expression: set x {2 + 3} expr [concat $x 5] That still has the faults, but now at least looks like it is doing something tricky. Which it is, given that the actual behaviour of the equivalent code in Tcl 8 is so messy. Another alternative (and a better one in this particular case) is: set x {2 + 3} expr {[expr $x] * 5} That also looks weird, but is better in that it only has the potential for trouble in one place and allows for better compilation of the outer parts of the expression. (Yes, it produces a different result from the other code. This is likely to be a good thing in almost all cases.)