X-Git-Url: https://git.xandkar.net/?p=hope.git;a=blobdiff_plain;f=src%2Fhope_fun.erl;h=96a39cda7fdf7be7f8f79d2ae8d3bcaac859d77a;hp=c2443cee8b7efb721098b69a9dd6268eb3bfac68;hb=0b6ed6177fd85b5dd1b6ceadb9c2e2560c6597d2;hpb=a3173cf2c4cd3aa3b7da033cac6b2d9e9ccd492a

diff --git a/src/hope_fun.erl b/src/hope_fun.erl
index c2443ce..96a39cd 100644
--- 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.