Create scaffolding for custom functions

app/src/main/kotlin/io/smnp/callable/function/CustomFunction.kt

@@ -0,0 +1,25 @@
+package io.smnp.callable.function
+
+import io.smnp.dsl.ast.model.node.FunctionDefinitionArgumentsNode
+import io.smnp.dsl.ast.model.node.FunctionDefinitionNode
+import io.smnp.dsl.ast.model.node.IdentifierNode
+import io.smnp.type.model.Value
+
+object CustomFunction {
+    fun create(node: FunctionDefinitionNode): Function {
+        val identifier = (node.identifier as IdentifierNode).token.rawValue
+
+        return object : Function(identifier) {
+            override fun define(new: FunctionDefinitionTool) {
+                val (_, argumentsNode, bodyNode) = node
+                val signature = FunctionSignatureParser.parseSignature(argumentsNode as FunctionDefinitionArgumentsNode)
+
+                new function signature define { env, args ->
+                    val boundArguments = FunctionEnvironmentProvider.provideEnvironment(argumentsNode, args, env)
+                    // TODO(Implement bodyNode evaluation)
+                    Value.void()
+                }
+            }
+        }
+    }
+}

app/src/main/kotlin/io/smnp/callable/function/FunctionEnvironmentProvider.kt

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+package io.smnp.callable.function
+
+import io.smnp.dsl.ast.model.node.FunctionDefinitionArgumentsNode
+import io.smnp.dsl.ast.model.node.IdentifierNode
+import io.smnp.dsl.ast.model.node.OptionalFunctionDefinitionArgumentNode
+import io.smnp.dsl.ast.model.node.RegularFunctionDefinitionArgumentNode
+import io.smnp.environment.Environment
+import io.smnp.error.ShouldNeverReachThisLineException
+import io.smnp.evaluation.evaluator.ExpressionEvaluator
+import io.smnp.type.model.Value
+
+object FunctionEnvironmentProvider {
+    fun provideEnvironment(signature: FunctionDefinitionArgumentsNode, actualArgs: List<Value>, environment: Environment): Map<String, Value> {
+        val evaluator = ExpressionEvaluator()
+        return signature.items.mapIndexed { index, node ->
+            when (node) {
+                is RegularFunctionDefinitionArgumentNode -> (node.identifier as IdentifierNode).token.rawValue to actualArgs[index]
+                is OptionalFunctionDefinitionArgumentNode -> (node.identifier as IdentifierNode).token.rawValue to evaluator.evaluate(
+                    node.defaultValue,
+                    environment
+                ).value!!
+                else -> throw ShouldNeverReachThisLineException()
+            }
+        }.toMap()
+    }
+}