[Haskell-cafe] Errors with Template Haskell

[Edsko de Vries]

The Template Haskell quotation monad (Q) has proper support for fail:

    module A where

    import Language.Haskell.TH

    foo :: Q Exp
    foo = fail "Custom compile error!"

and

    module B where

    import A

    main :: IO ()
    main = print $foo

gives

    B.hs:6:14:
        Custom compile error!
        In the first argument of `print', namely `$foo'
        In the expression: print ($foo)
        In an equation for `main': main = print ($foo)

-E

On Fri, Aug 9, 2013 at 9:48 AM, Jose A. Lopes <jabolopes at google.com> wrote:
> Hi,
>
> In Template Haskell, what is the proper way of signalling an error ?
>
> For example, you are generating code and you detect that a given
> parameter does not fulfill a precondition (e.g., String is empty), and
> you want to abort compilation with a descriptive error message.
>
> Thanks,
> Jose
>
> --
> Jose Antonio Lopes
> Ganeti Engineering
> Google Germany GmbH
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