Where are the magic rules for GHC assert?

Question

http://hackage.haskell.org/package/base-4.6.0.1/docs/src/GHC-Base.html#assert seems to define assert to be a no-op. Where's the logic that turns this into something else when assertions are enabled?

Answer 1

The comment above that function gives a hint:

-- Assertion function.  This simply ignores its boolean argument.
-- The compiler may rewrite it to @('assertError' line)@.

So, just use github code search and search for assertError: search results

This turns up the file RnExpr.lhs. Searching for assert in this file, you'll find the following code:

finishHsVar :: Name -> RnM (HsExpr Name, FreeVars)
-- Separated from rnExpr because it's also used
-- when renaming infix expressions
-- See Note [Adding the implicit parameter to 'assert']
finishHsVar name
 = do { this_mod <- getModule
      ; when (nameIsLocalOrFrom this_mod name) $
        checkThLocalName name

      ; ignore_asserts <- goptM Opt_IgnoreAsserts
      ; if ignore_asserts || not (name `hasKey` assertIdKey)
        then return (HsVar name, unitFV name)
        else do { e <- mkAssertErrorExpr
                ; return (e, unitFV name) } }

That's where it replaces assert by assertError, but only if assertions are enabled. assertError is defined in GHC.IO.Exception