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 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 Dienerstr. 12, 80331, München

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