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format_from_string and string_of_format #7799

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vicuna opened this issue May 31, 2018 · 5 comments

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commented May 31, 2018

Original bug ID: 7799
Reporter: gmelquiond
Assigned to: @nojb
Status: resolved (set by @nojb on 2018-06-07T07:29:19Z)
Resolution: fixed
Priority: normal
Severity: major
Version: 4.06.1
Category: standard library
Monitored by: @nojb @yakobowski

Bug description

I naively expect the functions format_from_string and string_of_format to be inverse of each other. But they are not, which leads to some obscure bugs when trying to use them. As far as I can tell, it is because format_from_string escapes double quotes but not backslashes before parsing the string. This is reproducible with all the versions of OCaml I have tested, from 4.02.3 to 4.07-beta.

Steps to reproduce

let test s = ignore (Scanf.format_from_string (string_of_format s) s);;
test "%s/%a";; (* OK )
test "\ ";; (
Exception: Scanf.Scan_failure "illegal escape character ' '". )
test "\x";; (
Exception: Scanf.Scan_failure "illegal escape character '"'". )
test "\x25s";; (
Exception: Scanf.Scan_failure "bad input: format type mismatch between "%s" and "\\x25s"". )
test "\"%s";; (
Exception: Scanf.Scan_failure "bad input: format type mismatch between "\\" and "\\\"%s"". )
test "\";; (
Exception: Scanf.Scan_failure "scanning of a String failed: premature end of file occurred before end of token". *)

@vicuna

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commented May 31, 2018

Comment author: @gasche

Indeed, it appears that the Scanf.format_of_string implementation, which has its own implementation of string escaping, is buggy. The following would work correctly:

let format_from_string s fmt =
Scanf.sscanf_format (Printf.sprintf "%S" s) fmt (fun s -> s)

I can fix this in the stdlib, but you can also use the code above as a workaround for yourself.

@vicuna

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commented May 31, 2018

Comment author: @gasche

Note: since the Big Format Change of 4.02, there is also a function CamlinternalFormat.format_of_string_format, with the same interface as Scanf.format_from_string, which I believe behaves correctly -- its implementation is more directly than Scanf's. CamlinternalFormat functions are internal and not meant for external usage, but this is also a workaround.

I'm curious, what is your use-case for using these functions?

@vicuna

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commented May 31, 2018

Comment author: gmelquiond

Sometimes, operator '^^' is not sufficient to manipulate format strings, so you have to go back to plain strings to benefit from a lot more functions. In fact, unless I missed it, there is not even a function that takes a nonliteral string free of '%' and turns it into a plain format string (except for format_of_string, obviously).

Anyway, thanks for the suggestion of using Scanf.sscanf_format. It is much nicer than the one I had come up for Why3, which was to escape all the backslashes before feeding it to format_from_string.

@vicuna

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commented Jun 6, 2018

Comment author: @nojb

Just as a follow-up, Scanf.format_from_string actually uses CamlinternalFormat.format_of_string_format. The problem is the implementation of escaping in Scanf.string_to_String which only escapes double quotes.

I submitted a PR with a variant of the fix suggested by Gabriel:

#1820

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commented Jun 7, 2018

Comment author: @nojb

PR merged

@vicuna vicuna closed this Jun 7, 2018

@vicuna vicuna added the stdlib label Mar 14, 2019

@vicuna vicuna added the bug label Mar 20, 2019

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