--- SLPJ: this was transferred from the TH branch
--- and I've forgotten what it was for... so I'll
--- comment it back out for now. It conflicts with
--- 'assert' in GHC.Prim
--- Assertion function. This simply ignores its boolean argument.
--- The compiler may rewrite it to (assertError line)
--- assert :: Bool -> a -> a
--- assert pred r = r
+-- Assertion function. This simply ignores its boolean argument.
+-- The compiler may rewrite it to @('assertError' line)@.
+
+-- | If the first argument evaluates to 'True', then the result is the
+-- second argument. Otherwise an 'AssertionFailed' exception is raised,
+-- containing a 'String' with the source file and line number of the
+-- call to 'assert'.
+--
+-- Assertions can normally be turned on or off with a compiler flag
+-- (for GHC, assertions are normally on unless the @-fignore-asserts@
+-- option is given). When assertions are turned off, the first
+-- argument to 'assert' is ignored, and the second argument is
+-- returned as the result.
+
+-- SLPJ: in 5.04 etc 'assert' is in GHC.Prim,
+-- but from Template Haskell onwards it's simply
+-- defined here in Base.lhs
+assert :: Bool -> a -> a
+assert pred r = r