CLHS: Section 22.3.5.2

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## 22.3.5.2 Tilde Less-Than-Sign: Logical Block

`~<...~:>`

If `~:>` is used to terminate a `~<...~>`, the directive is equivalent to a call to pprint-logical-block. The argument corresponding to the `~<...~:>` directive is treated in the same way as the list argument to pprint-logical-block, thereby providing automatic support for non-list arguments and the detection of circularity, sharing, and depth abbreviation. The portion of the control-string nested within the `~<...~:>` specifies the `:prefix` (or `:per-line-prefix`), `:suffix`, and body of the pprint-logical-block.

The control-string portion enclosed by `~<...~:>` can be divided into segments `~<`prefix`~;`body`~;`suffix`~:>` by `~;` directives. If the first section is terminated by `~@;`, it specifies a per-line prefix rather than a simple prefix. The prefix and suffix cannot contain format directives. An error is signaled if either the prefix or suffix fails to be a constant string or if the enclosed portion is divided into more than three segments.

If the enclosed portion is divided into only two segments, the suffix defaults to the null string. If the enclosed portion consists of only a single segment, both the prefix and the suffix default to the null string. If the colon modifier is used (i.e., `~:<...~:>`), the prefix and suffix default to `"("` and `")"` (respectively) instead of the null string.

The body segment can be any arbitrary format string. This format string is applied to the elements of the list corresponding to the `~<...~:>` directive as a whole. Elements are extracted from this list using pprint-pop, thereby providing automatic support for malformed lists, and the detection of circularity, sharing, and length abbreviation. Within the body segment, `~^` acts like pprint-exit-if-list-exhausted.

`~<...~:>` supports a feature not supported by pprint-logical-block. If `~:@>` is used to terminate the directive (i.e., `~<...~:@>`), then a fill-style conditional newline is automatically inserted after each group of blanks immediately contained in the body (except for blanks after a <Newline> directive). This makes it easy to achieve the equivalent of paragraph filling.

If the at-sign modifier is used with `~<...~:>`, the entire remaining argument list is passed to the directive as its argument. All of the remaining arguments are always consumed by `~@<...~:>`, even if they are not all used by the format string nested in the directive. Other than the difference in its argument, `~@<...~:>` is exactly the same as `~<...~:>` except that circularity detection is not applied if `~@<...~:>` is encountered at top level in a format string. This ensures that circularity detection is applied only to data lists, not to format argument lists.

`" . #`n`#"` is printed if circularity or sharing has to be indicated for its argument as a whole.

To a considerable extent, the basic form of the directive `~<...~>` is incompatible with the dynamic control of the arrangement of output by `~W`, `~_`, `~<...~:>`, `~I`, and `~:T`. As a result, an error is signaled if any of these directives is nested within `~<...~>`. Beyond this, an error is also signaled if the `~<...~:;...~>` form of `~<...~>` is used in the same format string with `~W`, `~_`, `~<...~:>`, `~I`, or `~:T`.

See also Section 22.3.6.2 (Tilde Less-Than-Sign: Justification).

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