diff --git a/packages/squiggle-lang/__tests__/Reducer/Reducer_functionTricks_test.res b/packages/squiggle-lang/__tests__/Reducer/Reducer_functionTricks_test.res
index 69e20067..23b4eaba 100644
--- a/packages/squiggle-lang/__tests__/Reducer/Reducer_functionTricks_test.res
+++ b/packages/squiggle-lang/__tests__/Reducer/Reducer_functionTricks_test.res
@@ -1,17 +1,42 @@
 open Jest
 open Reducer_TestHelpers
 
-Skip.describe("function trics", () => {
-  testEvalToBe("f(x,y)=x(y); f(f)", "????")    
-  testEvalToBe("f(x)=x(y); f(f)", "????")    
-  testEvalToBe("f(x)=x; f(f)", "????")    
+describe("Arity check", () => {
+  testEvalToBe("f(x,y) = x + y; f(1,2)", "Ok(3)")
+  testEvalToBe(
+    "f(x,y) = x + y; f(1)",
+    "Error(2 arguments expected. Instead 1 argument(s) were passed.)",
+  )
+  testEvalToBe(
+    "f(x,y) = x + y; f(1,2,3)",
+    "Error(2 arguments expected. Instead 3 argument(s) were passed.)",
+  )
+  testEvalToBe(
+    "f(x,y)=x+y; f(1,2,3,4)",
+    "Error(2 arguments expected. Instead 4 argument(s) were passed.)",
+  )
+  testEvalToBe(
+    "f(x,y)=x+y; f(1)",
+    "Error(2 arguments expected. Instead 1 argument(s) were passed.)",
+  )
+  testEvalToBe(
+    "f(x,y)=x(y); f(f)",
+    "Error(2 arguments expected. Instead 1 argument(s) were passed.)",
+  )
+  testEvalToBe("f(x)=x; f(f)", "Ok(lambda(x=>internal code))")
+  testEvalToBe(
+    "f(x,y)=x(y); f(z)",
+    "Error(2 arguments expected. Instead 1 argument(s) were passed.)",
+  )
+})
 
-  testEvalToBe("f(x,y)=x(y); f(z)", "????")
-  testEvalToBe("f(x,y)=x(y); f(2)", "????")  //prevent js error
-  testEvalToBe("f(x)=f(y)=2; f(2)", "????")  //prevent multiple assignment
-  testEvalToBe("f(x)=x+1; g(x)=f(x)+1;g(2)", "????") //TODO: f is not found
-  testEvalToBe("y=2;g(x)=y+1;g(2)", "????") //TODO : y is not found
-  testEvalToBe("y=2;g(x)=inspect(y)+1", "????") //TODO : 666
-  testEvalToBe("f(x,y)=x+y; f(1,2,3,4)", "????") //TODO : arity))
-  testEvalToBe("f(x,y)=x+y; f(1)", "????") //TODO : arity))
-})
\ No newline at end of file
+describe("function trics", () => {
+  testEvalToBe("f(x)=x(y); f(f)", "Error(y is not defined)")
+  testEvalToBe("f(x)=x; f(f)", "Ok(lambda(x=>internal code))")
+  testEvalToBe("f(x)=x(y); f(z)", "Error(y is not defined)")
+  MySkip.testEvalToBe("f(x)=x(y); f(2)", "????") //prevent js error
+  MySkip.testEvalToBe("f(x)=f(y)=2; f(2)", "????") //prevent multiple assignment
+  MySkip.testEvalToBe("f(x)=x+1; g(x)=f(x)+1;g(2)", "????") //TODO: f is not found
+  MySkip.testEvalToBe("y=2;g(x)=y+1;g(2)", "????") //TODO : y is not found
+  MySkip.testEvalToBe("y=2;g(x)=inspect(y)+1", "????") //TODO : 666
+})
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res
index 96b73fd2..b9eabe28 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res
@@ -1,7 +1,9 @@
 @genType
 type errorValue =
+  | REArityError(option<string>, int, int) //TODO: Binding a lambda to a variable should record the variable name in lambda for error reporting
   | REArrayIndexNotFound(string, int)
   | REAssignmentExpected
+  | REDistributionError(DistributionTypes.error)
   | REExpressionExpected
   | REFunctionExpected(string)
   | REJavaScriptExn(option<string>, option<string>) // Javascript Exception
@@ -9,7 +11,6 @@ type errorValue =
   | RERecordPropertyNotFound(string, string)
   | RESymbolNotFound(string)
   | RESyntaxError(string)
-  | REDistributionError(DistributionTypes.error)
   | RETodo(string) // To do
 
 type t = errorValue
@@ -17,6 +18,10 @@ type t = errorValue
 @genType
 let errorToString = err =>
   switch err {
+  | REArityError(_oFnName, arity, usedArity) =>
+    `${Js.String.make(arity)} arguments expected. Instead ${Js.String.make(
+        usedArity,
+      )} argument(s) were passed.`
   | REArrayIndexNotFound(msg, index) => `${msg}: ${Js.String.make(index)}`
   | REAssignmentExpected => "Assignment expected"
   | REExpressionExpected => "Expression expected"
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_ExpressionBuilder.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_ExpressionBuilder.res
index 989d2810..3aa0fcba 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_ExpressionBuilder.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_ExpressionBuilder.res
@@ -29,8 +29,8 @@ let eLambda = (parameters: array<string>, context, expr) =>
   BExpressionValue.EvLambda({
     parameters: parameters,
     context: context,
-    body: expr->castExpressionToInternalCode}
-  )->BExpressionT.EValue
+    body: expr->castExpressionToInternalCode,
+  })->BExpressionT.EValue
 
 let eNumber = aNumber => aNumber->BExpressionValue.EvNumber->BExpressionT.EValue
 
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_Lambda.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_Lambda.res
index 675e3bc3..55931cc7 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_Lambda.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_Lambda.res
@@ -1,7 +1,9 @@
 module Bindings = Reducer_Expression_Bindings
+module ErrorValue = Reducer_ErrorValue
 module ExpressionBuilder = Reducer_Expression_ExpressionBuilder
 module ExpressionT = Reducer_Expression_T
 module ExpressionValue = ReducerInterface.ExpressionValue
+module Result = Belt.Result
 
 type environment = ReducerInterface_ExpressionValue.environment
 type expression = ExpressionT.expression
@@ -11,25 +13,48 @@ type internalCode = ReducerInterface_ExpressionValue.internalCode
 
 external castInternalCodeToExpression: internalCode => expression = "%identity"
 
+let checkArity = (lambdaValue: ExpressionValue.lambdaValue, args: list<expressionValue>) => {
+  let argsLength = Belt.List.length(args)
+  let parametersLength = Js.Array2.length(lambdaValue.parameters)
+  if argsLength !== parametersLength {
+    ErrorValue.REArityError(None, parametersLength, argsLength)->Error
+  } else {
+    args->Ok
+  }
+}
+
+let checkIfReduced = (args: list<expressionValue>) =>
+  args->Belt.List.reduceReverse(Ok(list{}), (rAcc, arg) =>
+    rAcc->Result.flatMap(acc =>
+      switch arg {
+      | EvSymbol(symbol) => ErrorValue.RESymbolNotFound(symbol)->Error
+      | _ => list{arg, ...acc}->Ok
+      }
+    )
+  )
+
 let applyParametersToLambda = (
-  internal: internalCode,
-  parameters: array<string>,
-  args: list<expressionValue>,
-  context: externalBindings,
+  lambdaValue: ExpressionValue.lambdaValue,
+  args,
   environment,
   reducer: ExpressionT.reducerFn,
 ): result<expressionValue, 'e> => {
-  let expr = castInternalCodeToExpression(internal)
-  let parameterList = parameters->Belt.List.fromArray
-  let zippedParameterList = parameterList->Belt.List.zip(args)
-  let bindings = Belt.List.reduce(zippedParameterList, context->Bindings.fromExternalBindings, (
-    acc,
-    (variable, variableValue),
-  ) => acc->Belt.Map.String.set(variable, variableValue))
-  let newExpression = ExpressionBuilder.eBlock(list{expr})
-  reducer(newExpression, bindings, environment)
+  checkArity(lambdaValue, args)->Result.flatMap(args =>
+    checkIfReduced(args)->Result.flatMap(args => {
+      let expr = castInternalCodeToExpression(lambdaValue.body)
+      let parameterList = lambdaValue.parameters->Belt.List.fromArray
+      let zippedParameterList = parameterList->Belt.List.zip(args)
+      let bindings = Belt.List.reduce(
+        zippedParameterList,
+        lambdaValue.context->Bindings.fromExternalBindings,
+        (acc, (variable, variableValue)) => acc->Belt.Map.String.set(variable, variableValue),
+      )
+      let newExpression = ExpressionBuilder.eBlock(list{expr})
+      reducer(newExpression, bindings, environment)
+    })
+  )
 }
 
 let doLambdaCall = (lambdaValue: ExpressionValue.lambdaValue, args, environment, reducer) => {
-  applyParametersToLambda(lambdaValue.body, lambdaValue.parameters, args, lambdaValue.context, environment, reducer)
+  applyParametersToLambda(lambdaValue, args, environment, reducer)
 }
diff --git a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExpressionValue.res b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExpressionValue.res
index 4d6911af..5c9ee4b7 100644
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExpressionValue.res
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExpressionValue.res
@@ -45,8 +45,7 @@ let rec toString = aValue =>
     }
   | EvBool(aBool) => Js.String.make(aBool)
   | EvCall(fName) => `:${fName}`
-  | EvLambda(lambdaValue) =>
-    `lambda(${Js.Array2.toString(lambdaValue.parameters)}=>internal code)`
+  | EvLambda(lambdaValue) => `lambda(${Js.Array2.toString(lambdaValue.parameters)}=>internal code)`
   | EvNumber(aNumber) => Js.String.make(aNumber)
   | EvString(aString) => `'${aString}'`
   | EvSymbol(aString) => `:${aString}`