# Caveats The warnings and errors in this documents do not cover the complete expl3 language. The caveats currently include the following areas, among others: - Functions with “weird” (`w`) argument specifiers - Symbolic evaluation of expansion functions [@latexteam2024interfaces, sections 5.4–5.10] - Validation of parameters in (inline) functions (c.f. <#invalid-parameters-in-message-text> and <#incorrect-number-of-arguments-supplied-to-message>) - Shorthands such as `\~` and `\\` in message texts [@latexteam2024interfaces, sections 11.4 and 12.1.3] - Quotes in shell commands and file names [@latexteam2024interfaces, Section 10.7 and Chapter 12] - Functions used outside their intended context: - `\sort_return_*:` outside comparison code [@latexteam2024interfaces, Section 6.1] - `\prg_return_*:` outside conditional functions [@latexteam2024interfaces, Section 9.1] - Predicates (`\*_p:*`) outside boolean expressions [@latexteam2024interfaces, Section 9.3] - `\*_map_break:*` outside a corresponding mapping [@latexteam2024interfaces, sections 9.8] - `\msg_line_*:`, `\iow_char:N`, and `\iow_newline:` outside message text [@latexteam2024interfaces, sections 11.3 and 12.1.3] - `\iow_wrap_allow_break:` and `\iow_indent:n` outside wrapped message text [@latexteam2024interfaces, Section 12.1.4] - Token list and string variables without accessor functions `\tl_use:N` and `\str_use:N` - Boolean variable without an accessor function `\bool_to_str:N` outside boolean expressions [@latexteam2024interfaces, Section 21.4] - Integer variable without an accessor function `\int_use:N` outside integer or floating point expressions [@latexteam2024interfaces, Section 21.4] - Dimension variable without an accessor function `\dim_use:N` outside dimension or floating point expressions [@latexteam2024interfaces, Section 26.7] - Skip variable without an accessor function `\skip_use:N` outside skip or floating point expressions [@latexteam2024interfaces, Section 26.14] - Muskip variable without an accessor function `\muskip_use:N` outside muskip or floating point expressions [@latexteam2024interfaces, Section 26.21] - Floating point variable without an accessor function `\fp_use:N` outside floating point expressions [@latexteam2024interfaces, Section 29.3] - Box variable without accessor functions `\box_use(_drop)?:N` or `\[hv]box_unpack(_drop)?:N`, or without a measuring function `\box_(dp|ht|wd|ht_plus_dp):*` outside dimension or floating point expressions [@latexteam2024interfaces, sections 35.2 and 35.3] - Coffin variable without accessor function `\coffin_typeset:Nnnnn` outside dimension or floating point expressions [@latexteam2024interfaces, Section 36.4] - Lonely variables of other types that may or may not have accessor functions - Validation of literal expressions: - Comparison expressions in functions `\*_compare(_p:n|:nT?F?)` - Regular expressions and replacement text [@latexteam2024interfaces, sections 8.1 and 8.2] - Boolean expressions [@latexteam2024interfaces, Section 9.3] - Integer expressions and bases [@latexteam2024interfaces, sections 21.1 and 21.8] - Dimension, skip, and muskip expressions [@latexteam2024interfaces, Chapter 26] - Floating point expressions [@latexteam2024interfaces, Section 29.12] - Color expressions [@latexteam2024interfaces, Chapter 37.3] - Validation of naming schemes and member access: - String encoding and escaping [@latexteam2024interfaces, Section 18.1] - Key–value interfaces [@latexteam2024interfaces, Chapter 27]: - Are keys defined at the point of use or is the module or its subdivision set up to accept unknown keys? [@latexteam2024interfaces, sections 27.2, 27.5, and 27.6] - Are inheritance parents, choices, multi-choices, and groups used in a key definition defined at points of use? [@latexteam2024interfaces, sections 27.1, 27.3, and 27.7] - Floating-point symbolic expressions and user-defined functions [@latexteam2024interfaces, sections 29.6 and 29.7] - Names of bitset indexes [@latexteam2024interfaces, Section 31.1] - BCP-47 language tags [@latexteam2024interfaces, Section 34.2] - Color support [@latexteam2024interfaces, Chapter 37]: - Named colors [@latexteam2024interfaces, Section 37.4] - Color export targets [@latexteam2024interfaces, Section 37.8] - Color models and their families and params [@latexteam2024interfaces, sections 37.2 and 37.9] - Function `\file_input_stop:` not used on its own line [@latexteam2024interfaces, Section 12.2.3] - Exhaustively or fully expanding quarks and scan marks [@latexteam2024interfaces, Chapter 19] - Bounds checking for accessing constant sequences and other sequences where the number of items can be easily bounded such as integer and floating point arrays [@latexteam2024interfaces, chapters 28 and 30]: - Index checking functions `\*_range*:*` and `\*_item*:*` - Endless loop checking in functions `\*_step_*:*` [@latexteam2024interfaces, Section 21.7] - Number of symbols in a value-to-symbol mapping [@latexteam2024interfaces, Section 21.8] - Applying functions `\clist_remove_duplicates:N` and `\clist_if_in:*` to comma lists that contain `{`, `}`, or `*` [@latexteam2024interfaces, sections 23.3 and 23.4] - Incorrect parameters to function `\char_generate:nn` [@latexteam2024interfaces, Section 24.1] - Incorrect parameters to functions `\char_set_*code:nn` [@latexteam2024interfaces, Section 24.2] - Using implicit tokens `\c_catcode_(letter|other)_token` or the token list `\c_catcode_active_tl` [@latexteam2024interfaces, Section 24.3] - Validation of key–value interfaces [@latexteam2024interfaces, Chapter 27]: - Setting a key with some properties `.*_g?(set|put)*:*` should be validated similarly to calling the corresponding functions directly: Have the variables been declared, do they have the correct type, does the value have the correct type? - Do points of use always set keys with property `.value_required:n` and never set keys with property `.value_forbidden:n`? - Horizontal box operation on a vertical box or vice versa [@latexteam2024interfaces, Chapter 35], similarly for coffins - Statically checking the invariants in `\debug_assert:*` and, when they cannot be disproven, using them to infer information about the state of variables and constants. [@starynovotny2025adda] [@starynovotny2025addb]