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author | Egor Tensin <Egor.Tensin@gmail.com> | 2020-05-07 01:48:11 +0000 |
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committer | Egor Tensin <Egor.Tensin@gmail.com> | 2020-05-07 01:48:11 +0000 |
commit | adb9b3fe169a92ea2da7b8c804c8b91f966c21a4 (patch) | |
tree | fe340f4de0afa6763e220194d2e6bea3366b4c71 /_posts/2018-02-18-peculiar-indentation.md | |
parent | bind-mounts: rename the file (diff) | |
download | jekyll-theme-adb9b3fe169a92ea2da7b8c804c8b91f966c21a4.tar.gz jekyll-theme-adb9b3fe169a92ea2da7b8c804c8b91f966c21a4.zip |
remove everything except Jekyll theme stuff
I tagged the previous commit to explain what's going on.
Diffstat (limited to '')
-rw-r--r-- | _posts/2018-02-18-peculiar-indentation.md | 106 |
1 files changed, 0 insertions, 106 deletions
diff --git a/_posts/2018-02-18-peculiar-indentation.md b/_posts/2018-02-18-peculiar-indentation.md deleted file mode 100644 index 7832194..0000000 --- a/_posts/2018-02-18-peculiar-indentation.md +++ /dev/null @@ -1,106 +0,0 @@ ---- -title: Peculiar Haskell indentation -excerpt: > - An explanation for nasty `parse error`s I used to get for nested `do` - 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) -``` |