Implemented placeholder parameter #129

CHANGELOG.md

@@ -5,6 +5,7 @@
 - Improved GC allocations (#81)
 - Added wrapping of `Task` in `Future` (#64)
 - Added octal representation (e.g., `0o123`) for numbers
+- Added insertion points (`_`) to function calls (#129)
 - Added JSX syntax (#120)
 - Added default `jsx` and `html` function (#120)
 - Added events to `Engine` to handle uncaught errors (#121)

doc/language.md

@@ -270,4 +270,32 @@ cos(2.5) == cos()()(2.5)
 
 are therefore equal. In the previous example two calls without any arguments result in the function itself, such that the last call is operating on `cos` itself. Auto currying is applied to any standard function, custom created function (lambda expression), and wrapped .NET function.
 
-Auto currying allows to use functions to create functions without much ceremony and plays great together with the pipe operator, which expects functions requiring only a single argument on the right side.
+Auto currying allows to use functions to create functions without much ceremony and plays great together with the pipe operator, which expects functions requiring only a single argument on the right side.
+
+### Placeholders
+
+Calling a function with parameters supplied as `_` will result in a new function taking the number of used placeholders as arguments.
+
+Example:
+
+```
+var f = (x, y, z) => x + 2 * y + 3 * z
+```
+
+Classically, this could be called like `f(x, y, z)` or with auto-currying like `f(x, y)` or even `f(x)`. But what if we would like to have a function `g(y) = f(x, y, z)`? We would need to define this alias quite explicitly:
+
+```
+var g = (y) => f(1, y, 3)
+```
+
+With the placeholder parameter it can be written such as:
+
+```
+var g = f(1, _, 3)
+```
+
+The big advantage of this is that we get a simple way to pipe things:
+
+```
+4 | f(1, _, 3)
+```

src/Mages.Core/Ast/Walkers/OperationTreeWalker.cs

@@ -7,6 +7,7 @@
 using Mages.Core.Vm.Operations;
 using System;
 using System.Collections.Generic;
+using System.Linq;
 
 /// <summary>
 /// Represents the walker to create operations.
@@ -328,10 +329,12 @@ void ITreeWalker.Visit(ConditionalExpression expression)
     void ITreeWalker.Visit(CallExpression expression)
     {
         var assigning = _assigning;
+        var start = _operations.Count;
         _assigning = false;
 
         expression.Validate(this);
         expression.Arguments.Accept(this);
+        var end = _operations.Count;
         expression.Function.Accept(this);
 
         if (assigning)
@@ -340,7 +343,32 @@ void ITreeWalker.Visit(CallExpression expression)
         }
         else
         {
+            var extract = 0;
             _operations.Add(new GetcOperation(expression.Arguments.Count));
+
+            for (var i = start; i < end; i++)
+            {
+                if (_operations[i] is GetsOperation g && g.Name == "_")
+                {
+                    var c = extract++;
+                    _operations[i] = new GetsOperation($"_{c}");
+                }
+            }
+
+            if (extract > 0)
+            {
+                var parameters = new List<ParameterDefinition>();
+
+                for (var i = 0; i < extract; i++)
+                {
+                    var name = $"_{i}";
+                    _operations.Insert(start, new ArgOperation(extract - (i + 1), name));
+                    parameters.Add(new ParameterDefinition(name, true));
+                }
+
+                _operations.Insert(start + extract, ArgsOperation.Instance);
+                _operations.Add(ExtractFunction(false, parameters.ToArray(), start));
+            }
         }
 
         _assigning = assigning;

src/Mages.Core/Vm/Operations/DefOperation.cs

@@ -9,6 +9,8 @@ sealed class DefOperation(String name) : IOperation
 {
     private readonly String _name = name;
 
+    public String Name => _name;
+
     public void Invoke(IExecutionContext context)
     {
         var value = context.Pop();

src/Mages.Core/Vm/Operations/DelVarOperation.cs

@@ -10,6 +10,8 @@ sealed class DelVarOperation(String name) : IOperation
 {
     private readonly String _name = name;
 
+    public String Name => _name;
+
     public void Invoke(IExecutionContext context)
     {
         var result = context.Scope.Remove(_name);

src/Mages.Core/Vm/Operations/GetsOperation.cs

@@ -10,6 +10,8 @@ sealed class GetsOperation(String name) : IOperation
 {
     private readonly String _name = name;
 
+    public String Name => _name;
+
     public void Invoke(IExecutionContext context)
     {
         var value = context.Scope.GetProperty(_name);