Haskell/Alex: Warning: Tab character

Question

I'm writing a toy language in haskell. I'm using Alex to generate a scanner. Every time I build my project with cabal, I get the following warnings:

dist/build/optimiser/optimiser-tmp/Lexer.hs:465:1: Warning:
    Tab character

dist/build/optimiser/optimiser-tmp/Lexer.hs:466:1: Warning:
    Tab character

dist/build/optimiser/optimiser-tmp/Lexer.hs:467:1: Warning:
    Tab character

dist/build/optimiser/optimiser-tmp/Lexer.hs:471:1: Warning:
    Tab character

dist/build/optimiser/optimiser-tmp/Lexer.hs:472:1: Warning:
    Tab character

(plus about 10 more of these warnings)

It seems that the alex-generated scanner uses tabs instead of spaces, and ghc's default behaviour is to warn the user about this.

Is anyone aware of a way to either force alex to use spaces, or to suppress those warnings from ghc (via cabal)?

This warning is caused by a bug in Alex, which appears to have been fixed by Simon Marlow two weeks ago. See https://github.com/simonmar/alex/issues/61. Try to get the latest version of Alex. — jub0bs, May 14 '15 at 11:59
@Jubobs good to know, thanks for the heads-up. The update doesn't seem to have reached Hackage or my disribution's repos yet, but I can tolerate the issue until then. — AlexJ136, May 14 '15 at 12:55

score 10 · Accepted Answer · answered May 14 '15 at 14:51

10

As mentioned in the comments, this is supposedly fixed in alex master. Until you can use the fixed version, you can try adding

ghc-options: -fno-warn-tabs

to your .cabal file.

answered May 14 '15 at 14:51

Daniel Wagner

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Is it possible to make that flag apply only to the modules Alex generates? It looks like a heavy hammer. – dfeuer May 14 '15 at 16:18
In fact ... I am not convinced this was fixed. The commit you point to only removes tabs from the Alex source code, not from the code it generates. – dfeuer May 14 '15 at 16:23
That'll do nicely for the time being. Cheers. – AlexJ136 May 15 '15 at 09:31
@dfeuer Can't you manually ass `{-# GHC_OPTIONS #-}` to the top of the Alex input files? – MathematicalOrchid May 18 '15 at 15:39
4

@MathematicalOrchid That should be `OPTIONS_GHC`. So to be clear: `{-# OPTIONS_GHC -fno-warn-tabs #-}` should be at the top of the `.x`. – Josh Kirklin Jan 05 '16 at 10:27

Haskell/Alex: Warning: Tab character

1 Answers1