diff options
Diffstat (limited to '_posts')
-rw-r--r-- | _posts/2018-02-18-peculiar-indentation.md | 107 |
1 files changed, 107 insertions, 0 deletions
diff --git a/_posts/2018-02-18-peculiar-indentation.md b/_posts/2018-02-18-peculiar-indentation.md new file mode 100644 index 0000000..9c6930d --- /dev/null +++ b/_posts/2018-02-18-peculiar-indentation.md @@ -0,0 +1,107 @@ +--- +title: Peculiar Haskell indentation +layout: post +excerpt: > + An explanation of how Haskell indentation works inside <code>do</code> + blocks. +category: Haskell +custom_css: + - syntax.css +--- +I've fallen into a Haskell indentation pitfall. +I think it must be common, so I'm describing it here. + +The problem is that indentation rules in `do` blocks are not intuitive to me. +For example, the following function is valid Haskell syntax: + +```haskell +foo1 :: IO () +foo1 = + alloca $ \a -> + alloca $ \b -> + alloca $ \c -> do + poke a (1 :: Int) + poke b (1 :: Int) + poke c (1 :: Int) + return () +``` + +In fact, this funnier version is also OK: + +```haskell +foo2 :: IO () +foo2 = alloca $ \a -> + alloca $ \b -> + alloca $ \c -> do + poke a (1 :: Int) + poke b (1 :: Int) + poke c (1 :: Int) + return () +``` + +If you add an outer `do` however, things become a little more complicated. +For example, this is the valid version of the functions above with an outer +`do`: + +```haskell +foo3 :: IO () +foo3 = do + alloca $ \a -> + alloca $ \b -> + alloca $ \c -> do + poke a (1 :: Int) + poke b (1 :: Int) + poke c (1 :: Int) + return () +``` + +Notice the extra indentation for each of the `alloca`s. +When I tried to remove these seemingly excessive indents, GHC complained with +the usual `parse error (possibly incorrect indentation or mismatched +brackets)`. + +```haskell +foo4 :: IO () +foo4 = do + alloca $ \a -> + alloca $ \b -> + alloca $ \c -> do + poke a (1 :: Int) + poke b (1 :: Int) + poke c (1 :: Int) + return () +``` + +The truth is, the rules for desugaring `do` blocks are surprisingly simple and +literal. +GHC inserts semicolons according to the rules [found in the Wikibook]. +So it inserts semicolons between the `alloca`s on the same level, so `foo4` +becomes: + +```haskell +foo4 :: IO () +foo4 = do + { alloca $ \a -> + ; alloca $ \b -> + ; alloca $ \c -> do + { poke a (1 :: Int) + ; poke b (1 :: Int) + ; poke c (1 :: Int) + ; return () + } + } +``` + +[found in the Wikibook]: https://en.wikibooks.org/wiki/Haskell/Indentation#Explicit_characters_in_place_of_indentation + +The semicolons after `->` are clearly invalid Haskell syntax, hence the error. + +P.S. To compile the functions above, you need to include them in a module and +add proper imports, e.g. + +```haskell +module PeculiarIndentation where + +import Foreign.Marshal.Alloc (alloca) +import Foreign.Storable (poke) +``` |