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Add 'u' specifier to std.format #3970

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Add 'u' specifier to std.format #3970

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8f09b60
Add 'u' specifier to std.format
data-man Dec 22, 2019
5b9047c
Merge branch 'master' of https://github.com/ziglang/zig into utf8_fmt
data-man Dec 29, 2019
a985907
use 21 bits
data-man Dec 29, 2019
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27 changes: 27 additions & 0 deletions lib/std/fmt.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -76,6 +76,7 @@ fn peekIsAlign(comptime fmt: []const u8) bool {
/// - `d`: output numeric value in decimal notation
/// - `b`: output integer value in binary notation
/// - `c`: output integer as an ASCII character. Integer type must have 8 bits at max.
/// - `u`: output integer as an UTF-8 sequence. Integer type must have 21 bits at max.
/// - `*`: output the address of the value instead of the value itself.
///
/// If a formatted user type contains a function of the type
Expand Down Expand Up @@ -520,6 +521,12 @@ pub fn formatIntValue(
} else {
@compileError("Cannot print integer that is larger than 8 bits as a ascii");
}
} else if (comptime std.mem.eql(u8, fmt, "u")) {
if (@TypeOf(int_value).bit_count <= 21) {
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@daurnimator daurnimator Dec 29, 2019

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Does a comptime int pass this check?

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@data-man data-man Dec 29, 2019

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Copy-pasted from c modifier.

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@andrewrk andrewrk Dec 29, 2019

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I do want this to handle comptime_int please. The code for the c modifier is also incorrect if it cannot handle them. Until recently, due to limitations of the zig language, comptime_ints could not be passed to formatted printing. However that is now fixed, and comptime ints can be passed.

I would be OK with a non-explicit @compileError call here, allowing the @as to do the job. That would be my personal strategy to handle comptime_int correctly. Note also that the @as is redundant. Every parameter always gets type coerced to the parameter type.

return formatUtf8Codepoint(@as(u21, int_value), options, context, Errors, output);
} else {
@compileError("Cannot print integer that is larger than 21 bits as an UTF-8 sequence");
}
} else if (comptime std.mem.eql(u8, fmt, "b")) {
radix = 2;
uppercase = false;
Expand Down Expand Up @@ -587,6 +594,18 @@ pub fn formatAsciiChar(
return format(context, Errors, output, "\\x{x:0<2}", .{c});
}

pub fn formatUtf8Codepoint(
c: u21,
options: FormatOptions,
context: var,
comptime Errors: type,
output: fn (@TypeOf(context), []const u8) Errors!void,
) Errors!void {
var buf: [4]u8 = undefined;
const len = std.unicode.utf8Encode(c, buf[0..]) catch unreachable;
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@daurnimator daurnimator Dec 29, 2019

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why is this catch unreachable? what if someone e.g. passed in 0x1fffff

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@data-man data-man Dec 29, 2019

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attempt to unwrap error: CodepointTooLarge

Personally I'd prefer to have utf8EncodeUnsafe with 0xFFFD replacement.

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@andrewrk andrewrk Dec 29, 2019

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I'm OK with this function asserting that the codepoint fits, with a bit more defensive coding:

  • Document the assertion in doc comments of the function
  • Rather than catch unreachable, catch |err| switch (err) and then list the error(s) explicitly that are unreachable. This will cause compile errors if new errors are added to the set.

return output(context, @as(*const [4]u8, &buf)[0..len]);
}

pub fn formatBuf(
buf: []const u8,
options: FormatOptions,
Expand Down Expand Up @@ -1207,6 +1226,14 @@ test "int.specifier" {
const value: u8 = 'a';
try testFmt("u8: a\n", "u8: {c}\n", .{value});
}
{
const value: u8 = 'a';
try testFmt("UTF-8: a\n", "UTF-8: {u}\n", .{value});
}
{
const value: u21 = 0x1F310;
try testFmt("UTF-8: 🌐\n", "UTF-8: {u}\n", .{value});
}
{
const value: u8 = 0b1100;
try testFmt("u8: 0b1100\n", "u8: 0b{b}\n", .{value});
Expand Down