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)?

Protestantism

This warning is caused by a bug in Alex, which appears to have been fixed by Simon Marlow two weeks ago. See github.com/simonmar/alex/issues/61. Try to get the latest version of Alex. – Sass

@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. – Protestantism

Answer 1

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.

Edwinaedwine

Is it possible to make that flag apply only to the modules Alex generates? It looks like a heavy hammer. – Rinker

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. – Rinker

That'll do nicely for the time being. Cheers. – Protestantism

@Rinker Can't you manually ass {-# GHC_OPTIONS #-} to the top of the Alex input files? – Fantasy

@Fantasy That should be OPTIONS_GHC. So to be clear: {-# OPTIONS_GHC -fno-warn-tabs #-} should be at the top of the .x. – Traceytrachea