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+---
+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
+---
+A Haskell indentation pitfall I've fallen into.
+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.
+First, GHC inserts semicolons according to the rules described here:
+https://en.wikibooks.org/wiki/Haskell/Indentation#Explicit_characters_in_place_of_indentation.
+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 ()
+        }
+    }
+```
+
+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)
+```