Rule to recommend if-else construct instead of match clause

docs/content/how-tos/rule-configuration.md

@@ -132,3 +132,4 @@ The following rules can be specified for linting.
 - [FavourSingleton (FL0089)](rules/FL0089.html)
 - [NoAsyncRunSynchronouslyInLibrary (FL0090)](rules/FL0090.html)
 - [FavourNestedFunctions (FL0091)](rules/FL0091.html)
+- [FavourBasicControlFlow (FL0092)](rules/FL0092.html)

docs/content/how-tos/rules/FL0092.md

@@ -0,0 +1,30 @@
+---
+title: FL0092
+category: how-to
+hide_menu: true
+---
+
+# RecommendIfElseConstructOverMatch (FL0092)
+
+*Introduced in `0.26.12`*
+
+## Cause
+
+Use of match clause when it is used in basic control flow.
+
+## Rationale
+
+Match clauses are more adequate for complex pattern-matching scenarios.
+For simple comparisons, an if-then-else block makes terser code.
+
+## How To Fix
+
+Use if-else construct instead of match clause.
+
+## Rule Settings
+
+    {
+        "recommendIfElseConstructOverMatch": {
+            "enabled": false
+        }
+    }

src/FSharpLint.Core/Application/Configuration.fs

@@ -387,7 +387,8 @@ type ConventionsConfig =
       suggestUseAutoProperty:EnabledConfig option
       usedUnderscorePrefixedElements:EnabledConfig option
       ensureTailCallDiagnosticsInRecursiveFunctions:EnabledConfig option
-      favourNestedFunctions:EnabledConfig option }
+      favourNestedFunctions:EnabledConfig option
+      recommendIfElseConstructOverMatch:EnabledConfig option }
 with
     member this.Flatten() =
         Array.concat
@@ -416,6 +417,7 @@ with
                 this.ensureTailCallDiagnosticsInRecursiveFunctions |> Option.bind (constructRuleIfEnabled EnsureTailCallDiagnosticsInRecursiveFunctions.rule) |> Option.toArray
                 this.indexerAccessorStyleConsistency |> Option.bind (constructRuleWithConfig IndexerAccessorStyleConsistency.rule) |> Option.toArray
                 this.favourNestedFunctions |> Option.bind (constructRuleIfEnabled FavourNestedFunctions.rule) |> Option.toArray
+                this.recommendIfElseConstructOverMatch |> Option.bind (constructRuleIfEnabled RecommendIfElseConstructOverMatch.rule) |> Option.toArray
             |]
 
 [<Obsolete(ObsoleteMsg, ObsoleteWarnTreatAsError)>]
@@ -486,6 +488,7 @@ type Configuration =
       RedundantNewKeyword:EnabledConfig option
       FavourNonMutablePropertyInitialization:EnabledConfig option
       FavourReRaise:EnabledConfig option
+      RecommendIfElseConstructOverMatch:EnabledConfig option
       FavourStaticEmptyFields:EnabledConfig option
       AsyncExceptionWithoutReturn:EnabledConfig option
       UnneededRecKeyword:EnabledConfig option
@@ -589,6 +592,7 @@ with
         RedundantNewKeyword = None
         FavourNonMutablePropertyInitialization = None
         FavourReRaise = None
+        RecommendIfElseConstructOverMatch = None
         FavourStaticEmptyFields = None
         AsyncExceptionWithoutReturn = None
         UnneededRecKeyword = None
@@ -868,6 +872,7 @@ let flattenConfig (config:Configuration) =
                 config.FavourSingleton |> Option.bind (constructRuleIfEnabled FavourSingleton.rule)
                 config.NoAsyncRunSynchronouslyInLibrary |> Option.bind (constructRuleIfEnabled NoAsyncRunSynchronouslyInLibrary.rule)
                 config.FavourNestedFunctions |> Option.bind (constructRuleIfEnabled FavourNestedFunctions.rule)
+                config.RecommendIfElseConstructOverMatch |> Option.bind (constructRuleIfEnabled RecommendIfElseConstructOverMatch.rule)
             |]
 
     findDeprecation config deprecatedAllRules allRules

src/FSharpLint.Core/FSharpLint.Core.fsproj

@@ -61,6 +61,7 @@
     <Compile Include="Rules\Conventions\FailwithBadUsage.fs" />
     <Compile Include="Rules\Conventions\FavourSingleton.fs" />
     <Compile Include="Rules\Conventions\FavourNestedFunctions.fs" />
+    <Compile Include="Rules\Conventions\RecommendIfElseConstructOverMatch.fs" />
     <Compile Include="Rules\Conventions\SourceLength\SourceLengthHelper.fs" />
     <Compile Include="Rules\Conventions\SourceLength\MaxLinesInLambdaFunction.fs" />
     <Compile Include="Rules\Conventions\SourceLength\MaxLinesInMatchLambdaFunction.fs" />

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

@@ -0,0 +1,38 @@
+module FSharpLint.Rules.RecommendIfElseConstructOverMatch
+
+open FSharpLint.Framework
+open FSharpLint.Framework.Suggestion
+open FSharp.Compiler.Syntax
+open FSharp.Compiler.Text
+open FSharpLint.Framework.Ast
+open FSharpLint.Framework.Rules
+
+let runner args =
+    let isBasicControlFlow (synMatchClauses: List<SynMatchClause>) =
+        let (|Wildcard|_|) clause =
+            match clause with
+            | SynMatchClause(SynPat.Wild _, None, _, _, _, _) -> true
+            | _ -> false
+
+        match synMatchClauses with
+        | [ SynMatchClause(SynPat.Null(_), _, _, _, _, _); Wildcard ]
+        | [ SynMatchClause(SynPat.Const(_), _, _, _, _, _); Wildcard ]
+        | [ SynMatchClause(SynPat.Named(SynIdent.SynIdent(_), _, _, _), None, _, _, _, _); Wildcard ]
+        | [ SynMatchClause(SynPat.LongIdent(SynLongIdent(_), _, _, SynArgPats.Pats [], _, _), _, _, _, _, _); Wildcard ] -> 
+            true
+        | _ -> false
+
+    match args.AstNode with
+    | AstNode.Expression(SynExpr.Match (_, _, synMatchClauses, range, _)) when isBasicControlFlow synMatchClauses ->
+        { Range = range
+          Message = Resources.GetString "RecommendIfElseConstructOverMatch"
+          SuggestedFix = None
+          TypeChecks = List.empty }
+        |> Array.singleton
+    | _ -> Array.empty
+
+let rule =
+    { Name = "RecommendIfElseConstructOverMatch"
+      Identifier = Identifiers.RecommendIfElseConstructOverMatch
+      RuleConfig = { AstNodeRuleConfig.Runner = runner; Cleanup = ignore } }
+    |> AstNodeRule

src/FSharpLint.Core/Rules/Identifiers.fs

@@ -96,3 +96,4 @@ let IndexerAccessorStyleConsistency = identifier 88
 let FavourSingleton = identifier 89
 let NoAsyncRunSynchronouslyInLibrary = identifier 90
 let FavourNestedFunctions = identifier 91
+let RecommendIfElseConstructOverMatch = identifier 92

src/FSharpLint.Core/Text.resx

@@ -399,4 +399,7 @@
   <data name="RulesFavourNestedFunctions" xml:space="preserve">
     <value>Prefer using local functions over private module-level functions</value>
   </data>
+  <data name="RecommendIfElseConstructOverMatch" xml:space="preserve">
+    <value>Rather use simpler if-else construct instead of match clause.</value>
+  </data>
 </root>

src/FSharpLint.Core/fsharplint.json

@@ -299,6 +299,7 @@
     "avoidTooShortNames": { "enabled": false },
     "asyncExceptionWithoutReturn": { "enabled": false },
     "unneededRecKeyword": { "enabled": true },
+    "recommendIfElseConstructOverMatch": { "enabled": false },
     "indentation": {
         "enabled": false
     },

tests/FSharpLint.Core.Tests/FSharpLint.Core.Tests.fsproj

@@ -42,6 +42,7 @@
     <Compile Include="Rules\Conventions\NoPartialFunctions.fs" />
     <Compile Include="Rules\Conventions\FavourReRaise.fs" />
     <Compile Include="Rules\Conventions\FavourConsistentThis.fs" />
+    <Compile Include="Rules\Conventions\RecommendIfElseConstructOverMatch.fs" />
     <Compile Include="Rules\Conventions\AvoidTooShortNames.fs" />
     <Compile Include="Rules\Conventions\AvoidSinglePipeOperator.fs" />
     <Compile Include="Rules\Conventions\SuggestUseAutoProperty.fs" />

tests/FSharpLint.Core.Tests/Rules/Conventions/RecommendIfElseConstructOverMatch.fs

@@ -0,0 +1,75 @@
+module FSharpLint.Core.Tests.Rules.Conventions.RecommendIfElseConstructOverMatch
+
+open NUnit.Framework
+open FSharpLint.Rules
+open FSharpLint.Core.Tests
+
+[<TestFixture>]
+type TestConventionsRecommendIfElseConstructOverMatch() =
+    inherit TestAstNodeRuleBase.TestAstNodeRuleBase(RecommendIfElseConstructOverMatch.rule)
+
+
+    [<Test>]
+    member this.MoreThanTwoClausesShouldNotProduceError() =
+        this.Parse """
+match foo with
+| bar -> ()
+| baz -> ()
+| _ -> () """
+
+        Assert.IsTrue this.NoErrorsExist
+
+
+    [<Test>]
+    member this.TwoClausesWithoutWildcardShouldNotProduceError() =
+        this.Parse """
+match foo with
+| bar -> ()
+| baz -> () """
+
+        Assert.IsTrue this.NoErrorsExist
+
+    [<Test>]
+    member this.ThePresenceOfDUShouldNotProduceError() =
+        this.Parse """
+match foo with
+| Bar baz -> ()
+| _ -> () """
+
+        Assert.IsTrue this.NoErrorsExist
+
+    [<Test>]
+    member this.ThePresenceOfWhenClauseShouldNotProduceError() =
+        this.Parse """
+match foo with
+| bar when bar.Count = 0 -> ()
+| _ -> ()"""
+
+        Assert.IsTrue this.NoErrorsExist
+
+    [<Test>]
+    member this.TwoClausesWithWildcardShouldProduceError() =
+        this.Parse """
+match foo with
+| bar -> ()
+| _ -> () """
+
+        Assert.IsTrue this.ErrorsExist
+
+    [<Test>]
+    member this.TwoClausesWithNullAndWildcardShouldProduceError() =
+        this.Parse """
+match foo with
+| null -> ()
+| _ -> () """
+
+        Assert.IsTrue this.ErrorsExist
+
+    [<Test>]
+    member this.TwoClausesWithNullAndVariableShouldNotProduceError() =
+        this.Parse """
+match foo with
+| null -> ()
+| notNullFoo -> () """
+
+        Assert.IsTrue this.NoErrorsExist