3 ms·

Type systems that incorporate "nullables" allow programmers to have the concept of "nothing was returned" but force them to check it whenever they test for data

Type systems that incorporate "nullables" allow programmers to have the concept of "nothing was returned" but force them to check it whenever they test for data. Otherwise their programs don't compile (see the Maybe monad in Haskell). In a serious post about null, how could such a serious point be missed? For example, in Haskell: case objectMaybe of Just object -> do (your code that deals with `object` here) Nothing -> do (your code that deals with no object) Without the Nothing clause in the matching statement, this code will not compile.

VMG 12y ago
Not quite. sh-4.3$ cat test.hs main = let x = Nothing in case x of Just x -> putStrLn "something" sh-4.3$ ghc test.hs sh-4.3$ ./test test: test.hs:(2,11)-(3,40): Non-exhaustive patterns in case
- striking 12y ago
  Ah, so it will compile but break, thank you for mentioning that.
  - dllthomas 12y ago
    Not if you're building with -Wall -Werror.
  - VMG 12y ago
    ...which is not a real improvement compared to null-values
- dllthomas 12y ago
  Alternatively, sh-4.3~$ ghc -Wall -Werror test.hs [1 of 1] Compiling Main ( test.hs, test.o ) test.hs:1:1: Warning: Top-level binding with no type signature: main :: IO () test.hs:3:16: Warning: This binding for `x' shadows the existing binding bound at test.hs:1:12 test.hs:3:16: Warning: Defined but not used: `x' <no location info>: Failing due to -Werror.
  - VMG 12y ago
    This is where it gets weird on my system sh-4.3$ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.8.3 sh-4.3$ ghc -Wall -Werror test.hs sh-4.3$ Arch Linux 64bit. It works as expected with runhaskell though Edit: see answer below, thanks dilthomas!
    - dllthomas 12y ago
      Try touching test.hs (or add -fforce-recomp). By default, ghc won't rebuild if the object is newer than the source.
  - luchs 12y ago
    But it doesn't break because of the "Non-exhaustive patterns in case" that causes the runtime failure? These things look like they can be fixed without touching the case statement.
wcummings 12y ago
Amazing how close that is to valid Erlang
lostcolony 12y ago
So as others noted; this code will compile, but with a warning. Turn on warnings for incomplete patterns (-fwarn-incomplete-patterns), allow warnings to break compilation (-Werror), and you've got compile time safety. https://www.haskell.org/ghc/docs/7.0.4/html/users_guide/options-sanity.html https://www.haskell.org/ghc/docs/7.0.4/html/users_guide/opti... lists out what the flags do.

VMG 12y ago

Not quite. sh-4.3$ cat test.hs main = let x = Nothing in case x of Just x -> putStrLn "something" sh-4.3$ ghc test.hs sh-4.3$ ./test test: test.hs:(2,11)-(3,40): Non-exhaustive patterns in case

striking 12y ago
Ah, so it will compile but break, thank you for mentioning that.
- dllthomas 12y ago
  Not if you're building with -Wall -Werror.
- VMG 12y ago
  ...which is not a real improvement compared to null-values
dllthomas 12y ago
Alternatively, sh-4.3~$ ghc -Wall -Werror test.hs [1 of 1] Compiling Main ( test.hs, test.o ) test.hs:1:1: Warning: Top-level binding with no type signature: main :: IO () test.hs:3:16: Warning: This binding for `x' shadows the existing binding bound at test.hs:1:12 test.hs:3:16: Warning: Defined but not used: `x' <no location info>: Failing due to -Werror.
- VMG 12y ago
  This is where it gets weird on my system sh-4.3$ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.8.3 sh-4.3$ ghc -Wall -Werror test.hs sh-4.3$ Arch Linux 64bit. It works as expected with runhaskell though Edit: see answer below, thanks dilthomas!
  - dllthomas 12y ago
    Try touching test.hs (or add -fforce-recomp). By default, ghc won't rebuild if the object is newer than the source.
- luchs 12y ago
  But it doesn't break because of the "Non-exhaustive patterns in case" that causes the runtime failure? These things look like they can be fixed without touching the case statement.

wcummings 12y ago

Amazing how close that is to valid Erlang

lostcolony 12y ago

So as others noted; this code will compile, but with a warning. Turn on warnings for incomplete patterns (-fwarn-incomplete-patterns), allow warnings to break compilation (-Werror), and you've got compile time safety. https://www.haskell.org/ghc/docs/7.0.4/html/users_guide/options-sanity.html https://www.haskell.org/ghc/docs/7.0.4/html/users_guide/opti... lists out what the flags do.