Template Haskell - Static Assert

Question

I've written the following function:

staticAssert :: Bool -> Q [Dec]
staticAssert cond = case cond of
    True -> return []
    False -> fail "staticAssert failed"

Basically this evaluates condition at compile time and if it is false causes a compile error.

However, what I would like for staticAssert to instead of outputting "staticAssert failed", output the expression that failed the assertion.

If you write `$(fail "err")`, then the error will include the source file, line and column number, for example, I get `test.hs:8:3: err`. If you want to access the source position to print it in some custom way, us `Language.Haskell.TH.location` — user2407038, Feb 10 '14 at 09:25
You mean print the result of `location`? `$(location >>= fail . show)` (`Loc` doesn't have a show instance so you'll have to write or derive it yourself) — user2407038, Feb 10 '14 at 19:01
Which expression is that? Your function has type `Bool -> Q [Dec]`. — user2407038, Feb 12 '14 at 15:04
The bool part. I assume the signature will need to be changed. E.g. "Assertion failed: 2+2=5" — Clinton, Feb 13 '14 at 05:52
How about `assert bool str = if not bool then fail $ "Assertion failed: " ++ str else return []` — user2407038, Feb 13 '14 at 07:16
Then I'd have to write `assert (2 + 2 == 5) "(2 + 2 == 5)"`. This is error prone and violates Don't Repeat Yourself. — Clinton, Feb 14 '14 at 02:04

score 2 · Answer 1 · answered Feb 10 '14 at 09:23

2

It looks like you want location from Language.Haskell.TH.

answered Feb 10 '14 at 09:23

Carl

1 Answers1