f# - 为什么有些 FAKE 方法在目标函数中不起作用？

Question

考虑以下因素:

#r @"FakeLib.dll"

open Fake
open Fake.StringHelper
open Fake.ProcessHelper

Shell.Exec("mkdir","exampleDirectory")

Target "DoStuff" ( fun () -> 
    trace "Doing Stuff..."
)

Target "Clean" ( fun () -> 
    trace  "Cleaning..."
)

Target "Deploy" (fun () -> 
    trace  "Deploying..."
)

"DoStuff"
    ==>"Clean"
    ==>"Deploy"

RunTargetOrDefault "Deploy"

上面的脚本工作正常，但是当我像这样在目标中移动 Shell.Exec 时:

#r @"FakeLib.dll"

open Fake
open Fake.StringHelper
open Fake.ProcessHelper

Target "DoStuff" ( fun () -> 
    trace "Doing Stuff..."
    Shell.Exec("mkdir","exampleDirectory")
)

Target "Clean" ( fun () -> 
    trace  "Cleaning..."
)

Target "Deploy" (fun () -> 
    trace  "Deploying..."
)

"DoStuff"
    ==>"Clean"
    ==>"Deploy"

RunTargetOrDefault "Deploy"

我最终遇到了一个错误:

FsiEvaluationException:

Error:

        DeployScript.fsx(10,5): error FS0001: This expression was expected to have type
            unit
        but here has type
            int


Output: [Loading C:\Users\myusername\Desktop\SomeFolder\DeployScript.fsx]


Input: C:\Users\myusername\Desktop\SomeFolder\DeployScript.fsx
\Arguments:
  C:\fsi.exe

Exception: Yaaf.FSharp.Scripting.FsiEvaluationException: Error while compiling or executing fsharp snippet. ---> System.Exception: Operation failed. The error text has been print the error stream. To return the corresponding FSharpErrorInfo use the EvalInteractionNonThrowing, EvalScriptNonThrowing or EvalExpressionNonThrowing
   at Microsoft.FSharp.Compiler.Interactive.Shell.FsiEvaluationSession.commitResult[a,b](FSharpChoice`2 res)
   at Microsoft.FSharp.Compiler.Interactive.Shell.FsiEvaluationSession.EvalScript(String filePath)
   at Yaaf.FSharp.Scripting.Helper.evalScript@1303.Invoke(String arg00) in C:\code\fake\paket-files\matthid\Yaaf.FSharp.Scripting\src\source\Yaaf.FSharp.Scripting\YaafFSharpScripting.fs:line 1303
   at Yaaf.FSharp.Scripting.Helper.save_@1276-2.Invoke(Unit unitVar0) in C:\code\fake\paket-files\matthid\Yaaf.FSharp.Scripting\src\source\Yaaf.FSharp.Scripting\YaafFSharpScripting.fs:line 1277
   at Yaaf.FSharp.Scripting.Helper.consoleCapture[a](TextWriter out, TextWriter err, FSharpFunc`2 f) in C:\code\fake\paket-files\matthid\Yaaf.FSharp.Scripting\src\source\Yaaf.FSharp.Scripting\YaafFSharpScripting.fs:line 1221
   at Yaaf.FSharp.Scripting.Helper.redirectOut@1247[a](Boolean preventStdOut, OutStreamHelper out, OutStreamHelper err, FSharpFunc`2 f) in C:\code\fake\paket-files\matthid\Yaaf.FSharp.Scripting\src\source\Yaaf.FSharp.Scripting\YaafFSharpScripting.fs:line 1254
   at Yaaf.FSharp.Scripting.Helper.save_@1275-1.Invoke(String text) in C:\code\fake\paket-files\matthid\Yaaf.FSharp.Scripting\src\source\Yaaf.FSharp.Scripting\YaafFSharpScripting.fs:line 1276
   --- End of inner exception stack trace ---
   at Yaaf.FSharp.Scripting.Helper.save_@1275-1.Invoke(String text) in C:\code\fake\paket-files\matthid\Yaaf.FSharp.Scripting\src\source\Yaaf.FSharp.Scripting\YaafFSharpScripting.fs:line 1284
   at Yaaf.FSharp.Scripting.Helper.session@1306.Yaaf-FSharp-Scripting-IFsiSession-EvalScriptWithOutput(String ) in C:\code\fake\paket-files\matthid\Yaaf.FSharp.Scripting\src\source\Yaaf.FSharp.Scripting\YaafFSharpScripting.fs:line 1308
   at Fake.FSIHelper.runScriptUncached(Boolean useCache, String scriptPath, IEnumerable`1 fsiOptions, Boolean printDetails, CacheInfo cacheInfo, TextWriter out, TextWriter err) in C:\code\fake\src\app\FakeLib\FSIHelper.fs:line 471
DeployScript.fsx(10,5): error FS0001: This expression was expected to have type

这里的要点是我试图仅在特定目标下执行 shell 命令。不要注意 mkdir 命令，因为我实际上想使用 schtasks 命令。为了简单起见，我只是使用 mkdir 。也许我在这里遗漏了一个细微差别。我还注意到使用 environVarOrFail 时也会出现同样的行为。预先感谢所有回复的人。

Answer 1

Shell.Exec 返回一个 int，但 Target 需要一个返回 unit 的函数。

试试这个:

let returnUnit() = ()
let return42() = 42

Target "One" returnUnit // works
Target "Two" return42 // fails: expected to have type unit, but here has type int

目标“Two”无法编译，因为它的第二个参数是一个返回 int 的函数，而预期是返回 unit 的函数。

此错误是为了防止您忘记函数返回值:编译器会帮助您告诉您“嘿，您确定不想对这个 int 执行任何操作吗？那么您为什么首先调用该函数？”

然而，有时丢弃返回值是有意义的。例如，在这种特殊情况下，您调用 Shell.Exec 是因为它的副作用，而您对它的返回值不感兴趣。这种情况发生得非常频繁，以至于有一个特殊的函数可以应对它 - ignore。

Target "DoStuff" ( fun () -> 
    trace "Doing Stuff..."
    Shell.Exec("mkdir","exampleDirectory") |> ignore // compiles fine
)

该函数接受任意值，将其丢弃，然后返回一个单位。通过将其应用于Shell.Exec的结果，您可以告诉编译器:“是的，我确定我不需要这个值，请忽略它。”

<小时/>

也就是说，我强烈建议使用专用函数而不是通用系统调用。这将使您的脚本更具可读性，甚至可以帮助您及早发现偶尔的错误。例如，要创建目录，请使用 FileUtils.mkdir:

Target "DoStuff" ( fun () -> 
    trace "Doing Stuff..."
    FileUtils.mkdir "exampleDirectory"  // no need for `ignore`, mkdir already returns `unit`
)