:sprint var是sprint_即使在被评估之后。

Question

我读过Why :sprint always prints a "_"?，但似乎遇到了其他的问题。

ghci> sum = foldl (+) 0
ghci> let total = sum [1..1000000]
ghci> :sprint total
total = _ -- This is expected due to lazy eval
ghci> print total
500000500000
ghci> :sprint total
total = _ -- This is NOT expected since total has been evaluated.
ghci> :set -XMonomorphismRestriction -- recommended by one of the other answers.
ghci> :sprint total
total = _
ghci> print total
50000005000000
ghci> :sprint total
total = _
ghci> :sprint sum
sum = _
ghci> ints = [1..5]
ghci> :sprint ints
ints = _
ghci> print ints
[1,2,3,4,5]
ghci> :sprint ints
ints = [1,2,3,4,5] -- Yay!

有用的信息：

ghci> :show
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:

和

$ ghci --version
The Glorious Glasgow Haskell Compilation System, version 9.0.2

那么问题是:为什么，我能做些什么来“解决”这个问题呢？我和https://andrew.gibiansky.com/blog/haskell/haskell-syntax/一起读书

Answer 1

启用total扩展后，尝试定义MonomorphismRestriction。

ghci> :set -XMonomorphismRestriction 
ghci> sum = foldl (+) 0
ghci> let total = sum [1..1000000]
ghci> :sprint total
total = _
ghci> print total
500000500000
ghci> :sprint total
total = 500000500000

或者您可以给total指定一个特定类型(没有MonomorphismRestriction扩展)。

ghci> sum = foldl (+) 0
ghci> let total = sum [1..1000000] :: Int
ghci> :sprint total
total = _
ghci> print total
500000500000
ghci> :sprint total
total = 500000500000

问题是，在NoMonomorphismRestriction (默认)下，total的类型将是(Num a, Enum a) => a。如果值的类型是多态的，GHCi每次都会对其进行评估，就像这样。因此，:sprintf无法打印计算值。

当您给它一个特定的类型时，:sprintf可以打印计算值。请注意，total的类型将成为与MonomorphismRestriction一起的Integer。