Implement function composition.

[hope.git] / src / hope_fun.erl

diff --git a/src/hope_fun.erl b/src/hope_fun.erl
index c2443ce..96a39cd 100644 (file)
--- a/src/hope_fun.erl
+++ b/src/hope_fun.erl
@@ -3,6 +3,9 @@
 -export(
     [ id/1
     , curry/1
+    , compose/1  % alias for compose_right/1
+    , compose_right/1
+    , compose_left/1
     ]).
 
 -spec id(A) ->
@@ -22,3 +25,26 @@ curry(F, Args, 0) ->
     apply(F, lists:reverse(Args));
 curry(F, Args, Arity) ->
     fun (X) -> curry(F, [X | Args], Arity - 1) end.
+
+-spec compose([fun((A) -> B)]) ->
+    fun((A) -> B).
+compose(Fs) ->
+    compose_right(Fs).
+
+-spec compose_right([fun((A) -> B)]) ->
+    fun((A) -> B).
+compose_right(Fs) ->
+    compose_given_fold(Fs, fun lists:foldr/3).
+
+-spec compose_left([fun((A) -> B)]) ->
+    fun((A) -> B).
+compose_left(Fs) ->
+    compose_given_fold(Fs, fun lists:foldl/3).
+
+
+%% ============================================================================
+
+-spec compose_given_fold([fun((A) -> B)], fun((fun((A, B) -> C), B, [A]) -> C)) ->
+    fun((A) -> C).
+compose_given_fold(Fs, Fold) ->
+    fun (X) -> Fold(fun (F, Y) -> F(Y) end, X, Fs) end.