Impoverished error reporting for TH-generated haskell

[kquick]

Given a fairly simple parser for "key=value" style input names_params.txt and using the compileDDLWith TH-generated parser, there is not much error information provided on parse failures.

ghci> runDParser $ pMain $ newInput "test" $ encodeUtf8 " min-length = 5; max-length=8; min-longth=3"
Left (ParseError {peSource = FromSystem, peLoc = "", peInput = Stream { off  = 31, len  = 11, name = "test"}, peMsg = "", peStack = []})

Compare this to:

$ cabal run exe:daedalus run -- names_params.ddl -i <(echo " min-length = 5; max-length=8; min-longth=3")
--- Parse error:
[offset: 36]
  • expected 'e' while matching "min-length"
  • see grammar at: names_params.ddl:25:5--25:19
  • context:
      • names_params.ddl:4:10--4:28
      • names_params.ddl:13:69--13:80
      • names_params.ddl:13:76--13:80
      • minLength
      • names_params.ddl:20:17--20:47
  • input trace:
      /dev/fd/63
        0--36
File context:
Length: 40 (0x28) bytes
0000:                2d 6c 65 6e  67 74 68 20  3d 20 35 3b       -length = 5;
0010:   20 6d 61 78  2d 6c 65 6e  67 74 68 3d  38 3b 20 6d    max-length=8; m
0020:   69 6e 2d 6c  6f 6e 67 74  68 3d 33 0a     in-longth=3.

Notably:

• peLoc always seems to be a null string for errorLevel=1 and is inscrutable for errorLevel greater than one (for the above: peLoc = "DETERMINIZE 1 Fully"). I had expected this to provide location information in the input; currently I'm extracting that from peInput.off, which may be the recommended method, but I'm unsure as peLoc has no documentation.
• peStack always seems to be an empty list for errorLevel=1, but is not particularly helpful for errorLevel greater than one (for the above: peStack = [[":ManyBody_113"],[":Many_114",":Many_114","names_params.ddl:5:5--5:39:Many_114"]], where only the last entry is somewhat useful).
• peMsg is sometimes a null string (as seen above), and seems unaffected by errorLevel values.