考虑shell :: String -> CreateProcess
。例如:
Prelude System.IO System.Process> shell "pwd"
CreateProcess {cmdspec = ShellCommand "pwd", cwd = Nothing, env = Nothing, std_in = Inherit, std_out = Inherit, std_err = Inherit, close_fds = False, create_group = False, delegate_ctlc = False, detach_console = False, create_new_console = False, new_session = False, child_group = Nothing, child_user = Nothing, use_process_jobs = False}
我正在尝试修改CreateProcess
的记录参数,但收到REPL错误:
Prelude System.IO System.Process> shell "pwd" { cwd = "/home" }
<interactive>:7:7: error:
• Couldn't match type ‘CreateProcess’ with ‘[Char]’
Expected type: String
Actual type: CreateProcess
• In the first argument of ‘shell’, namely ‘"pwd" {cwd = "/home"}’
In the expression: shell "pwd" {cwd = "/home"}
In an equation for ‘it’: it = shell "pwd" {cwd = "/home"}
<interactive>:7:21: error:
• Couldn't match expected type ‘Maybe FilePath’
with actual type ‘[Char]’
• In the ‘cwd’ field of a record
In the first argument of ‘shell’, namely ‘"pwd" {cwd = "/home"}’
In the expression: shell "pwd" {cwd = "/home"}
您的代码中有两个问题。
cwd
具有类型Maybe String
,而不仅仅是String
,您需要显式使用Just
构造函数。{}
附加到最接近的参数,在这种情况下,这是"cwd"
字符串。但是,您要指定函数调用结果的字段。这样写代码对我有用:
ghci> (shell "cwd") { cwd = Just "/home" }
CreateProcess
{ cmdspec = ShellCommand "cwd"
, cwd = Just "/home"
, env = Nothing
, std_in = Inherit
, std_out = Inherit
, std_err = Inherit
, close_fds = False
, create_group = False
, delegate_ctlc = False
, detach_console = False
, create_new_console = False
, new_session = False
, child_group = Nothing
, child_user = Nothing
, use_process_jobs = False
}