Unicode output in GHC

Unicode output is somewhat broken in GHC as a whole. We should fix it properly.

Most output is generated by the Pretty module. Pretty has two ways to output:

• printLeftRender, which is used when the rendering mode is LeftMode.

  This method uses the BufWrite module to speed up output. For FastStrings,

  the output will be in UTF-8, for strings and other characters the output

  takes the low 8 bits of each character.

• printDoc, when used in modes other than LeftMode (e.g. for things like

  error messages and -ddump), calls hPutStr for strings which uses the

  prevailing encoding on stdout. However, it calls hPutFS for FastStrings,

  which always emits UTF-8.

• In GHCi, there is an additional layer due to Haskeline, which pipes all the

  output through its own decoder (or tries to, I think there are cases not

  covered).

This is all a bit of a mess.

We should be using the Unicode layer in the IO library for all encoding/decoding now. I suggest that:

• we leave printLeftRender alone. It is used for printing things like the

  .s file, and never outputs any Unicode characters because everything is

  Z-encoded.

• printDoc, instead of hPutFS, should use hPutStr . decodeFS

• We get rid of the Haskeline decoding layer.

However, this will introduce a regression on Windows, because the Haskeline encoding layer currently does code-page encoding. Judah has mentioned looking at doing code-page encoding in the GHC IO library, so let's see what happens there.

Once this is done, we can do #2507 (closed) (quotation characters in error messages).

Trac metadata

Trac field	Value
Version	6.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
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