Add Weak Alias RFC (#13)

Author: lzybkr

1-Draft/RFC0007-Weak-Aliases.md

@@ -0,0 +1,101 @@
+---
+RFC: 0007
+Author: Jason Shirk
+Status: Draft
+Area: Command Resolution
+---
+
+# Weak Aliases
+
+This RFC is an attempt to address the communities desire to [remove the aliases `curl` and `wget`]((https://github.com/PowerShell/PowerShell/pull/1901)) from Windows PowerShell.
+
+## Motivation
+
+The introduction of the aliases `curl` and `wget` happened in PowerShell V3 and was an unintentional breaking change.
+Regretfully, removing these aliases now would also be a breaking change.
+
+An analysis of scripts from various sources was performed to understand the usage patterns of `curl` in PowerShell scripts.
+The source of scripts included the Windows code base, GitHub and a corpus of scripts collected by the PowerShell team from various sources like poshcode.org.
+
+There is a [gist](https://gist.github.com/lzybkr/3cd091334355f381d1d6ee7acfad5a48) with the code I used to analyze the scripts from GitHub.
+
+My analysis concludes that `curl` calls `Invoke-WebRequest` approximately twice as often as `curl.exe`.
+
+In the Windows code base, usage of `curl` was not common, but was primarily used as an alias for `Invoke-WebRequest`.
+
+I analyzed 667 scripts on GitHub that contained the string `curl`.
+
+In 465 scripts, `curl` was not used as a command name.
+
+In 124 scripts, my analysis concluded that `curl.exe` was the intended command, but 3 specific scripts appeared many times in the data set. Eliminating those duplicates, we end up with 45 scripts using `curl.exe`.
+
+The remaining scripts are assumed to use `Invoke-WebRequest` - approximately 75 scripts.
+
+Removing the aliases breaks scripts in another unexpected way as well.
+Approximately 10 or so scripts remove the alias, presumably to call `curl.exe`, e.g.
+```powershell
+rm alias:curl
+```
+If the alias does not exist, this will result in an error, possibly breaking the script in an unexpected way.
+
+There are other scenarios where removal of the aliases cause undue difficulties for our customers.
+
+* PowerShell usage in compiled code - more difficult to discover
+* Windows PowerShell updates as part of Windows, possibly breaking *users* of scripts that don't know how to fix the script
+
+The conclusion of this analysis is that removing the aliases would cause enough problems that we should prefer another solution.
+
+## Specification
+
+This proposal is inspired by the notion of a [weak symbol](https://en.wikipedia.org/wiki/Weak_symbol) used by C/C++ compilers in the ELF and COFF object formats.
+
+In PowerShell, an alias has the highest order of command precedence.
+This proposal introduces a weak alias which would have the lowest order of command precedence.
+
+A weak alias can be created in various ways - both in C# and PowerShell:
+```C#
+    var alias = new SessionStateAliasEntry("curl", "Invoke-webRequest");
+    alias.Weak = true;
+    // Add alias to InitialSessionState
+```
+
+```powershell
+New-Alias -Name curl -Value Invoke-WebRequest -Weak
+```
+
+Command resolution changes as follows:
+
+1. During the search for aliases, if a weak alias is found, it is saved and the search resumes with looking for functions, cmdlets, and external commands.
+2. If no other commands are found, the saved weak alias is used.
+
+Note that this lookup describes a long standing confusing feature where `Get-*` commands may be invoked without the `Get-` prefix.
+With the introduction of weak aliases, we can codify this quirk of command resolution by creating a weak alias for all `Get-*` cmdlets, e.g.
+
+```powershell
+New-Alias -Name Process -Value Get-Process -Weak
+New-Alias -Name ChildItem -Value Get-ChildItem -Weak
+# etc
+```
+
+This also helps with discoverability when `Get-Command` returns these weak aliases.
+
+## Alternate Proposals and Considerations
+
+### Considerations
+
+Adding weak aliases for `curl` and `wget` could still be considered a breaking change.
+A script that means to use `Invoke-WebRequest` may stop working correctly if `curl.exe` is installed on a system.
+
+### Alternate Proposals
+
+Another way to implement a weak alias is to allow multiple definitions, e.g.
+
+```powershell
+New-Alias -Name curl -Definition 'curl.exe','Invoke-WebRequest'
+```
+
+Resolution would try each definition in the order specified.
+This design suffers a small problem of knowing the exact extension for the external command, which might be undesirable if the external command is often wrapped in another script, e.g. `curl.cmd`.
+
+There are some suggestions to choose the appropiate target command based on parameters.
+This suggestion has some significant flaws as in some cases, it is impossible to distinguish which target command was desired.