Custom prompt hooks protected from later overwriting

vendor/profile.ps1

@@ -46,16 +46,23 @@ try {
     $env:Path += $(";" + $env:CMDER_ROOT + "\vendor\git-for-windows\bin")
 }
 
-try {
-    Import-Module -Name "posh-git" -ErrorAction Stop >$null
-    $gitStatus = $true
-} catch {
-    Write-Warning "Missing git support, install posh-git with 'Install-Module posh-git' and restart cmder."
-    $gitStatus = $false
+$gitLoaded = $false
+function Import-Git($Loaded){
+    if($Loaded) { return }
+    $GitModule = Get-Module -Name Posh-Git -ListAvailable
+    if($GitModule | select version | where version -le ([version]"0.6.1.20160330")){
+        Import-Module Posh-Git > $null
+    }
+    if(-not ($GitModule) ) {
+        Write-Warning "Missing git support, install posh-git with 'Install-Module posh-git' and restart cmder."
+    }
+    # Make sure we only run once by alawys returning true
+    return $true
 }
 
 function checkGit($Path) {
     if (Test-Path -Path (Join-Path $Path '.git') ) {
+        $gitLoaded = Import-Git $gitLoaded
         Write-VcsStatus
         return
     }
@@ -65,24 +72,6 @@ function checkGit($Path) {
     }
 }
 
-# Set up a Cmder prompt, adding the git prompt parts inside git repos
-function global:prompt {
-    $realLASTEXITCODE = $LASTEXITCODE
-    $Host.UI.RawUI.ForegroundColor = "White"
-    Write-Host $pwd.ProviderPath -NoNewLine -ForegroundColor Green
-    if($gitStatus){
-        checkGit($pwd.ProviderPath)
-    }
-    $global:LASTEXITCODE = $realLASTEXITCODE
-    Write-Host "`nλ" -NoNewLine -ForegroundColor "DarkGray"
-    return " "
-}
-
-# Load special features come from posh-git
-if ($gitStatus) {
-    Start-SshAgent -Quiet
-}
-
 # Move to the wanted location
 # This is either a env variable set by the user or the result of
 # cmder.exe setting this variable due to a commandline argument or a "cmder here"
@@ -110,11 +99,83 @@ foreach ($x in ls *.ps1) {
 }
 popd
 
+#
+# Prompt Section
+#   Users should modify their user-profile.ps1 as it will be safe from updates.
+#
+
+# Pre assign the hooks so the first run of cmder gets a working prompt.
+[ScriptBlock]$PrePrompt = {}
+[ScriptBlock]$PostPrompt = {}
+[ScriptBlock]$CmderPrompt = {
+    $Host.UI.RawUI.ForegroundColor = "White"
+    Microsoft.PowerShell.Utility\Write-Host $pwd.ProviderPath -NoNewLine -ForegroundColor Green
+    checkGit($pwd.ProviderPath)
+}
+
 $CmderUserProfilePath = Join-Path $env:CMDER_ROOT "config\user-profile.ps1"
 if(Test-Path $CmderUserProfilePath) {
     # Create this file and place your own command in there.
     . "$CmderUserProfilePath"
 } else {
-    Write-Host "Creating user startup file: $CmderUserProfilePath"
-    "# Use this file to run your own startup commands" | Out-File $CmderUserProfilePath
+# This multiline string cannot be indented, for this reason I've not indented the whole block
+
+Write-Host -BackgroundColor Darkgreen -ForegroundColor White "First Run: Creating user startup file: $CmderUserProfilePath"
+
+$UserProfileTemplate = @'
+# Use this file to run your own startup commands
+
+## Prompt Customization
+<#
+.SYNTAX
+    <PrePrompt><CMDER DEFAULT>
+    λ <PostPrompt> <repl input>
+.EXAMPLE
+    <PrePrompt>N:\Documents\src\cmder [master]
+    λ <PostPrompt> |
+#>
+
+[ScriptBlock]$PrePrompt = {
+
+}
+
+# Replace the cmder prompt entirely with this.
+# [ScriptBlock]$CmderPrompt = {}
+
+[ScriptBlock]$PostPrompt = {
+
 }
+
+## <Continue to add your own>
+
+
+'@
+
+New-Item -ItemType File -Path $CmderUserProfilePath -Value $UserProfileTemplate > $null
+
+}
+
+# Once Created these code blocks cannot be overwritten
+Set-Item -Path function:\PrePrompt   -Value $PrePrompt   -Options Constant
+Set-Item -Path function:\CmderPrompt -Value $CmderPrompt -Options Constant
+Set-Item -Path function:\PostPrompt  -Value $PostPrompt  -Options Constant
+
+<#
+This scriptblock runs every time the prompt is returned.
+Explicitly use functions from MS namespace to protect from being overridden in the user session.
+Custom prompt functions are loaded in as constants to get the same behaviour
+#>
+[ScriptBlock]$Prompt = {
+    $realLASTEXITCODE = $LASTEXITCODE
+    $host.UI.RawUI.WindowTitle = Microsoft.PowerShell.Management\Split-Path $pwd.ProviderPath -Leaf
+    PrePrompt | Microsoft.PowerShell.Utility\Write-Host -NoNewline
+    CmderPrompt
+    Microsoft.PowerShell.Utility\Write-Host "`nλ " -NoNewLine -ForegroundColor "DarkGray"
+    PostPrompt | Microsoft.PowerShell.Utility\Write-Host -NoNewline
+    $global:LASTEXITCODE = $realLASTEXITCODE
+    return " "
+}
+
+# Functions can be made constant only at creation time
+# ReadOnly at least requires `-force` to be overwritten
+Set-Item -Path function:\prompt  -Value $Prompt  -Options ReadOnly