AvoidSinglePipeOperator,docs: improved rule 77

Changed rule AvoidSinglePipeOperator to not apply to functions
with more than one argument.

Updated the docs to reflect this change and give rationale for
it.

Author: webwarrior-ws

docs/content/how-tos/rules/FL0077.md

@@ -17,6 +17,10 @@ Use of the pipe operator when only one invocation is employed.
 When the use of the pipe operator happens for a single invocation (instead of a multiple one, which would add a chain of them, usually staggered across 
 many lines), it doesn't aid readibility.
 
+This rule does not apply to functions with more than one argument. The reason for that is primarily application of higher-order functions
+such as `map` or `iter`, for which type inference may break without using pipe operator. And it also it helps to have the collection,
+which is iterated over, to be on top.
+
 Example of single pipe operator usage:
 
 ```fsharp
@@ -34,6 +38,13 @@ let someFunc someParam =
     |> yetAnotherFunc
 ```
 
+Example of single pipe applied to function with several arguments:
+
+```fsharp
+projectsInOrder 
+    |> Seq.map(fun proj -> proj.AbsolutePath |> FileInfo)
+```
+
 ## How To Fix
 
 Just use a normal function call, without the need for any operator:

src/FSharpLint.Core/Rules/Conventions/AvoidSinglePipeOperator.fs

@@ -52,8 +52,13 @@ let runner (args: AstNodeRuleParams) =
 
     let error =
         match args.AstNode with
-        | AstNode.Expression(SynExpr.App(_exprAtomicFlag, _isInfix, funcExpr, _argExpr, _range)) ->
-            checkExpr funcExpr (args.GetParents args.NodeIndex)
+        | AstNode.Expression(SynExpr.App(_exprAtomicFlag, _isInfix, funcExpr, argExpr, _range)) ->
+            match argExpr with
+            | SynExpr.App(_) ->
+                // function has extra arguments
+                Array.empty
+            | _ ->
+                checkExpr funcExpr (args.GetParents args.NodeIndex)
         | _ ->
             Array.empty