Don't fail when rendering an Unbounded layout of a grouped Line

[sjakobi]

Fixes Doesn't fix #91.

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TODO:

• Add a more complex test case
• Check whether any documentation needs updating or clarification.

[sjakobi]

I was wondering whether we need to worry about SFail appearing elsewhere than the beginning of a left Union branch. At least fuse doesn't seem to "hide" the Fail in a (Union Fail x) though:

https://github.com/quchen/prettyprinter/blob/4d51362d7ea8bca0fe9e7ffec2ccd8fcd0065c0d/prettyprinter/src/Data/Text/Prettyprint/Doc/Internal.hs#L1407

[quchen]

isFail is False if the document fails, but that’s the same thing the two fitting predicates (for layoutPretty and layoutSmart) yield. What does the explicit isFail clause change?

[sjakobi]

I've explained this in #91 (comment).

The short version is that both fitting predicates shortcut to True when the maxWidth is Nothing – which is the case with Unbounded layout options.

[sjakobi]

This doesn't seem to fix the issue completely:

If I change dhall's layoutOpts to use Unbounded and run its test suite with this branch, I still see a bunch of these errors:

Exception: »SFail« must not appear in a rendered »SimpleDocStream«. This is a bug in the layout algorithm! Please report this as a bug

[sjakobi]

Maybe we should instead remove the shortcuts from the fitting predicates…

[sjakobi]

For reference, these are some of of the test failures:

      ./tests/format/letNewlineComments:                                                                                       FAIL
      ./tests/format/escapeSingleQuotedOpenInterpolation:                                                                      FAIL
      ./tests/format/issue1413:                                                                                                FAIL
      ./tests/format/importAccess:                                                                          
      ./tests/format/letComments:                                                                                              FAIL
      ./tests/format/innerMultiline:                                                                                           FAIL
    Issue #126:                                                                                                                FAIL
  tutorial
    Interpolation
      Example #1:                                                                                                              FAIL

[sjakobi]

Actually I wonder whether the current design with Unbounded as alternative to AvailablePageWidth makes sense at all. The entire difference between layoutSmart and layoutPretty lies in their fitting predicates – and with Unbounded you completely turn them off! layoutSmart (LayoutOptions Unbounded) and layoutPretty (LayoutOptions Unbounded) will always produce the same layout!

IMHO it would be better to have a separate function layoutUnbounded that handles Unions simply by checking whether the first alternative results in SFail.

[sjakobi]

Superseded by #97.