TIP 652: Remove "string is unicode" and Tcl_CharIsUnicode"

Author:		Nathan Coulter <[email protected]>
State:		Final
Type:		Project
Vote:		Done
Created:	26-Dec-2022
Tcl-Version:	8.7
Vote-Summary:  6 / 0 / 0
Votes-For:     BG, DP, JN, KW, MC, SL
Votes-Against: none
Votes-Present: none

Abstract and Specification

Remove string is Unicode and Tcl_UniCharIsUnicode(), introduced in TIP 597.


The only use of [string is unicode] is to determine whether a string can be encoded into a Unicode transformation format, either utf-8, utf-16, or utf-32. Tcl has never needed a [string is big5], [string is shiftjs] or any other [string is someencodinghere]. There is also no need for [string is unicode]. To determine whether a given string can be encoded into a given encoding, it is sufficient to attempt to perform the encoding using the "strict" profile. A failure to encode provides the same answer that [string is unicode] would have.

[string is unicode] fails in its stated purpose. According to TIP 597,

The string is unicode command can be used to check if the "utf-8"/"utf-16" encodings would deliver valid output, ...

This is not true:

set text \U03fffe
string is unicode $text;# -> 0
binary scan [encoding convertto utf-16 $text] H* hex
set hex ;# -> fdff

The problem is that according to TIP 597, in addition to the surrogate characters, the return value is also 0 for the 66 noncharacters, U+??FFFE - U+??FFFF and U+FDD0 - U+FDEF. This means that string is unicode and Tcl_UniCharIsUnicode() can not be used to check whether the data could be encoded into one of the Unicode encoding forms.

The Unicode specification makes it clear that noncharacters may be encoded into an encoding form. First, there is definition 79:

A Unicode encoding form assigns each Unicode scalar value to a unique code unit sequence.

The specification then declares:

To ensure that the mapping for a Unicode encoding form is one-to-one, all Unicode scalar values, including those corresponding to noncharacter code points and unassigned code points, must be mapped to unique code unit sequences. Note that this requirement does not extend to high-surrogate and low-surrogate code points, which are excluded by definition from the set of Unicode scalar values.

The Private-Use Characters, Noncharacters & Sentinels FAQ states,

Q: Are noncharacters invalid in Unicode strings and UTFs?

Absolutely not. Noncharacters do not cause a Unicode string to be ill-formed in any UTF. This can be seen explicitly in the table above, where every noncharacter code point has a well-formed representation in UTF-32, in UTF-16, and in UTF-8. An implementation which converts noncharacter code points between one UTF representation and another must preserve these values correctly. The fact that they are called "noncharacters" and are not intended for open interchange does not mean that they are somehow illegal or invalid code points which make strings containing them invalid.

As a response to issue Tcl 9: "illegal byte sequence" ?! all checks for noncharacters were removed in commit cbaa5e70167db75b. Therefore, [string is unicode] is already obsolete, having fallen behind the reality of the implementation. The only thing string is unicode now does is check for surrogate code points.

Tcl_UniCharIsUnicode() is also not useful. If some encoding functionality is to be exposed at the C level, the equivalent of encoding convertto could be provided.


An implementation is provided in the branch, "tip-652".


Copyright © 2023, Nathan Coulter. All rights reserved.


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