Implementing basic functions (#3554)

The language specification suggests to add [five basic functions into the standard library](https://github.com/enso-org/design/blob/wip/wd/enso-spec/epics/enso-spec-1.0/05.%20Functions.md#useful-functions-in-the-standard-library). `identity`, `flip`, `const`, `curry` & `uncurry`.

# Important Notes
The new functions are being added into existing `Function.enso` file. That may not be the best place, but it is not clear from the [design spec](https://github.com/enso-org/design/blob/wip/wd/enso-spec/epics/enso-spec-1.0/05.%20Functions.md#useful-functions-in-the-standard-library) how they are supposed to be imported. I can move them wherever needed.

There is a documentation provided for each of the functions, but I am not sure how to verify it is correct. Do we generate the documentation for stdlib somehow?

CHANGELOG.md

@@ -147,6 +147,7 @@
 - [Added `File_Format.Delimited` support to `Table.write` for new files.][3528]
 - [Adjusted `Database.connect` API to new design.][3542]
 - [Added `File_Format.Excel` support to `Table.write` for new files.][3551]
+- [identity,const,flip,curry,uncurry functions][3554]
 - [Added append support for `File_Format.Excel`.][3558]
 - [Added support for custom encodings in `File_Format.Delimited` writing.][3564]
 - [Allow filtering caught error type in `Error.catch`.][3574]
@@ -236,6 +237,7 @@
 [3542]: https://github.com/enso-org/enso/pull/3542
 [3551]: https://github.com/enso-org/enso/pull/3551
 [3552]: https://github.com/enso-org/enso/pull/3552
+[3554]: https://github.com/enso-org/enso/pull/3554
 [3558]: https://github.com/enso-org/enso/pull/3558
 [3564]: https://github.com/enso-org/enso/pull/3564
 [3574]: https://github.com/enso-org/enso/pull/3574

distribution/lib/Standard/Base/0.0.0-dev/src/Function.enso

@@ -1,3 +1,5 @@
+import Standard.Base.Data.Vector
+
 # Function types.
 type Function
 
@@ -7,3 +9,56 @@ type Function
        the this argument.
     @Builtin_Type
     type Function
+
+
+## An identity function which returns the provided argument.
+
+   Arguments:
+   - x: the value to return.
+
+   > Example
+     five = Function.identity 5 # returns number 5
+
+identity : a -> a
+identity x = x
+
+## Flips the first two arguments of a function. Returns function that
+   takes two arguments, but in opposite order.
+
+   Arguments:
+   - f function that takes two arguments
+
+   > Example
+    IO.println <| Function.flip (+) "world" "hello" # Prints 'helloworld'
+
+flip : (a -> b -> c) -> (b -> a -> c)
+flip f = (x -> y -> f y x)
+
+
+## Creates a function which drops its input and returns the provided value instead.
+   The expression const a is the same as \_ -> a.
+
+   Arguments:
+   - x constant value to return
+
+   > Example
+    IO.println <| [1, 2, 3].map (Function.const 7) # Prints '[7, 7, 7]'
+
+const : a -> b -> a
+const x _ = x
+
+## Converts a single-argument function accepting a pair of elements into a multi-argument one.
+
+   Arguments:
+   - f function accepting pair of values
+
+curry : ([a, b] -> c) -> (a -> b -> c)
+curry f = x -> y -> f [x, y]
+
+## Converts a multi-argument function into a single-argument one accepting a pair of elements.
+
+   Arguments:
+   - f function accepting multiple arguments
+
+uncurry : (a -> b -> c) -> ([a, b] -> c)
+uncurry f = (pair -> f pair.head pair.second)

engine/runtime/src/main/scala/org/enso/compiler/context/SuggestionBuilder.scala

@@ -403,6 +403,8 @@ final class SuggestionBuilder[A: IndexedSource](val source: A) {
           val tdef = resolveTypeName(bindings, typeName)
             .getOrElse(TypeArg.Value(QualifiedName.simpleName(typeName)))
           args :+ tdef
+        case seq: IR.Application.Literal.Sequence =>
+          seq.items.foldLeft(args)((a, t) => go(t, a))
         case _ =>
           args
       }

test/Tests/src/Data/Function_Spec.enso

@@ -0,0 +1,43 @@
+from Standard.Base import all
+import Standard.Base.Data.Numbers
+
+import Standard.Test
+from Standard.Base.Data.Ordering import Equal, Less, Greater
+
+spec =
+    Test.group "identity" <|
+        Test.specify "identity on number" <|
+            (identity 5) . should_equal 5
+
+        Test.specify "identity on text" <|
+            (identity '5') . should_equal '5'
+
+        Test.specify "identity on boolean" <|
+            (identity False) . should_equal False
+
+    Test.group "flip" <|
+        Test.specify "flip on number" <|
+            (flip (-) 2 5) . should_equal 3
+
+        Test.specify "flip on text" <|
+            (flip (+) "world" "hello") . should_equal "helloworld"
+
+    Test.group "const" <|
+        Test.specify "const on number" <|
+            two = const 2
+            two 5 . should_equal 2
+
+    Test.group "curry" <|
+        Test.specify "curry on number list" <|
+            sum = x -> x.fold 0 (+)
+            sum [1, 2, 3, 4] . should_equal 10
+            plus = curry sum
+            plus 6 3 . should_equal 9
+
+    Test.group "uncurry" <|
+        Test.specify "uncurry on number list" <|
+            times = uncurry (*)
+            times [6, 7] . should_equal 42
+
+
+main = Test.Suite.run_main spec

test/Tests/src/Main.enso

@@ -20,6 +20,7 @@ import project.Semantic.R_Interop_Spec
 
 import project.Data.Array_Spec
 import project.Data.Bool_Spec
+import project.Data.Function_Spec
 import project.Data.Interval_Spec
 import project.Data.Json_Spec
 import project.Data.List_Spec
@@ -66,6 +67,7 @@ main = Test.Suite.run_main <|
     Any_Spec.spec
     Array_Spec.spec
     Bool_Spec.spec
+    Function_Spec.spec
     Case_Spec.spec
     Conversion_Spec.spec
     Deep_Export_Spec.spec