haskell-jp / questions #86

sumCollects :: (Collects c1, Collects c2, Elem c1 ~ Elem c2) => c1 -> c2 -> c2
sumCollects c1 c2 = foldr insert c2 (toList c1)

class Collects ce where
  type Elem ce
  empty  :: ce
  insert :: Elem ce -> ce -> ce
  member :: Elem ce -> ce -> Bool
  toList :: ce -> [Elem ce]

sumCollects c1 c2 = foldr insert c2 (toList c1)

PersistEntityBackend val ~ SqlBackend

$ ghci
GHCi, version 8.6.5:   :? for help
Prelude> let x = 123
Prelude> return (x :: Int, x :: Float)
(123,123.0)

$ cat test.hs
test = do
  let x = 123
  return (x :: Int, x :: Float)
$ ghci
Prelude> :load "test.hs"
[1 of 1] Compiling Main             ( test.hs, interpreted )

test.hs:3:21: error:
    • Couldn't match expected type 'Float' with actual type 'Int'
    • In the expression: x :: Float
      In the first argument of 'return', namely '(x :: Int, x :: Float)'
      In a stmt of a 'do' block: return (x :: Int, x :: Float)
  |
3 |   return (x :: Int, x :: Float)
  |                     ^
Failed, no modules loaded.
Prelude> :set
options currently set: none.
base language is: Haskell2010
with the following modifiers:
  -XNoDatatypeContexts
  -XNondecreasingIndentation
GHCi-specific dynamic flag settings:
other dynamic, non-language, flag settings:
  -fexternal-dynamic-refs
  -fignore-optim-changes
  -fignore-hpc-changes
  -fimplicit-import-qualified
warning settings:
Prelude>

test :: IO (Int,Double)
test = do
  let { x :: Num a => a ; x = 1 }
  let y = (x,x) :: (Int,Double)
  return y

@naohaq @igrep

The restriction is turned on by default in compiled modules, and turned off by default at the GHCi prompt (since GHC 7.8.1).

ghcのperformGCはGCの完了まで待たずに、実行をブロックしないものと思っていますが

As it turns out, `randomR` and consequently `random` are oblivious to the concrete type that they are generating and all of the integral numbers go through the arbitrary precision `Integer` in order to produce the value in a desired range.

従ってmajor GCを2回走らせてみてオブジェクトが解放されるか確認してみるといいかもしれません。

over myLens <$> (ioAction) <*> data

type Lens s t a b = forall f. Functor f => (a -> f b) -> s -> f t 

use $1, $2, etc.
https://www.postgresql.org/docs/12/sql-prepare.html

.etc