計算機科学のブログ

-- クイックチェック9-1
remove :: (a -> Bool) -> [a] -> [a]
remove _ [] = []
remove test (x : xs) =
  if test x
    then remove test xs
    else x : remove test xs

-- クイックチェック9-2
myProduct :: (Foldable t, Num b) => t b -> b
myProduct xs = foldl (*) 1 xs

% ghci
GHCi, version 8.10.4: https://www.haskell.org/ghc/  :? for help
macro 'doc' overwrites builtin command.  Use ':def!' to overwrite.
(0.00 secs, 0 bytes)
(0.00 secs, 0 bytes)
Loaded GHCi configuration from /.../.ghc/ghci.conf
Prelude
λ> :load sample01
[1 of 1] Compiling Main             ( sample01.hs, interpreted )
Ok, one module loaded.
(0.01 secs,)
*Main
λ> remove even []
[]
it :: Integral a => [a]
(0.11 secs, 63,048 bytes)
*Main
λ> remove even [1]
[1]
it :: Integral a => [a]
(0.00 secs, 61,992 bytes)
*Main
λ> remove even [2]
[]
it :: Integral a => [a]
(0.00 secs, 61,088 bytes)
*Main
λ> remove even [1..10]
[1,3,5,7,9]
it :: Integral a => [a]
(0.01 secs, 71,472 bytes)
*Main
λ> :load sample01
[1 of 1] Compiling Main             ( sample01.hs, interpreted )

sample01.hs:10:14: error:
    Not in scope: type constructor or class ‘GHC.Types.Any’
    No module named ‘GHC.Types’ is imported.
   |
10 | myProduct :: GHC.Types.Any Integer -> Integer
   |              ^^^^^^^^^^^^^
Failed, no modules loaded.
(0.03 secs,)
Prelude
λ> :load sample01
[1 of 1] Compiling Main             ( sample01.hs, interpreted )

sample01.hs:10:13: error:
    • Ambiguous type variable ‘t0’ arising from a use of ‘foldl’
      prevents the constraint ‘(Foldable t0)’ from being solved.
      Relevant bindings include
        myProduct :: t0 Integer -> Integer (bound at sample01.hs:10:1)
      Probable fix: use a type annotation to specify what ‘t0’ should be.
      These potential instances exist:
        instance Foldable (Either a) -- Defined in ‘Data.Foldable’
        instance Foldable Maybe -- Defined in ‘Data.Foldable’
        instance Foldable ((,) a) -- Defined in ‘Data.Foldable’
        ...plus one other
        ...plus 26 instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In the expression: foldl (*) 1
      In an equation for ‘myProduct’: myProduct = foldl (*) 1
   |
10 | myProduct = foldl (*) 1
   |             ^^^^^^^^^^^
Failed, no modules loaded.
(0.07 secs,)
Prelude
λ> :load sample01
[1 of 1] Compiling Main             ( sample01.hs, interpreted )
Ok, one module loaded.
(0.00 secs,)
*Main
λ> myProduct []
1
it :: Num b => b
(0.04 secs, 59,784 bytes)
*Main
λ> myProduct [1]
1
it :: Num b => b
(0.00 secs, 59,904 bytes)
*Main
λ> myProduct [2

<interactive>:11:13: error:
    parse error (possibly incorrect indentation or mismatched brackets)
*Main
λ> myProduct [2]
2
it :: Num b => b
(0.00 secs, 59,904 bytes)
*Main
λ> myProduct [2, 3]
6
it :: Num b => b
(0.00 secs, 60,040 bytes)
*Main
λ> myProduct [1..10]
3628800
it :: (Num b, Enum b) => b
(0.00 secs, 65,608 bytes)
*Main
λ> :quit 
Leaving GHCi.
%