[#4] Implement styles composition (#6)

* Implement styles composition

* Move last to Util.Option

lib/Styling.ml

@@ -34,6 +34,23 @@ let create
   { foreground; background; bold; dim; italic; underlined }
 ;;
 
+let fg : [< Color.t ] -> t = fun foreground -> create ~foreground ()
+let bg : [< Color.t ] -> t = fun background -> create ~background ()
+let bold : t = { none with bold = true }
+let dim : t = { none with dim = true }
+let italic : t = { none with italic = true }
+let underlined : t = { none with underlined = true }
+
+let ( & ) left right =
+  let foreground = Util.Option.last left.foreground right.foreground in
+  let background = Util.Option.last left.background right.background in
+  let bold = left.bold || right.bold in
+  let dim = left.dim || right.dim in
+  let italic = left.italic || right.italic in
+  let underlined = left.underlined || right.underlined in
+  { foreground; background; bold; dim; italic; underlined }
+;;
+
 let make_sgr_sequence (inner : string) : string = "\x1b[" ^ inner ^ "m"
 
 let add_color_to_buffer

lib/Styling.mli

@@ -22,6 +22,44 @@ val create
   -> unit
   -> t
 
+(** [fg color] is a convenient constructor for creating composable styles.
+Creates a style with the specified foreground color only. See [&] for usage examples. *)
+val fg : [< Color.t ] -> t
+
+(** [fg color] is a convenient constructor for creating composable styles.
+Creates a style with the specified background color only. See [&] for usage
+examples. *)
+val bg : [< Color.t ] -> t
+
+(** [bold] is a convenient constructor for creating composable styles.
+Creates a bold style only. See [&] for usage examples. *)
+val bold : t
+
+(** [dim] is a convenient constructor for creating composable styles.
+Creates a dim style only. See [&] for usage examples. *)
+val dim : t
+
+(** [italic] is a convenient constructor for creating composable styles.
+Creates an italic style only. See [&] for usage examples. *)
+val italic : t
+
+(** [underlined] is a convenient constructor for creating composable styles.
+Creates an underline style only. See [&] for usage examples. *)
+val underlined : t
+
+(** [left & right] combines two styles with the following rules:
+
+- For colors, [right] is taken if it's present, otherwise [left]
+- For boolean fields, the {b or} combination of both is applied
+
+Usage example:
+
+{[
+  let my_style = Styling.(default_style & fg yellow & bg black & bold) in
+]}
+*)
+val ( & ) : t -> t -> t
+
 (** [to_ansi styling] renders the [styling] to an ANSI escape
     sequence as a string. *)
 val to_ansi : t -> string

lib/Util.ml

@@ -77,7 +77,7 @@ module Triplet = struct
     | _, _, (Error _ as error) -> error
   ;;
 
-  (** [all_error triplet] returns a version of [triplet] wrapped in 
+  (** [all_error triplet] returns a version of [triplet] wrapped in
       [result], which is of the [Error] variant if every member
       is also of the [Error] variant, otherwise it is whichever
       is the first [Ok]. *)
@@ -94,7 +94,7 @@ module Triplet = struct
   (** [any_ok triplet] returns a version of [triplet] wrapped
       in [result], which is the first [Ok] encountered if there
       is any, otherwise it is the triplet wrapped in [Error].
-      
+
       It is the same as [all_error], but it is semantically
       useful to have it as a separate function. *)
   let any_ok = all_error
@@ -116,3 +116,13 @@ module Triplet = struct
     map func func func
   ;;
 end
+
+module Option = struct
+  include Option
+
+  let last left right =
+    match right with
+    | Some _ -> right
+    | None -> left
+  ;;
+end