maybe monad and friends

maybe - return a tuple of (Nothing) or (Just,value)
maybemap - apply map function when maybe has a value and wrap in another maybe; else when nothing return nothing
maybevalue - return value of maybe; else when nothing return optional default args

src/fun.sh

@@ -358,3 +358,43 @@ call() {
     local args=$@
     tup $f $args
 }
+
+maybe() {
+  if [[ $# -eq 0 ]]; then
+    local arg
+    read arg
+    maybe "$arg"
+  else
+    local x="$*"
+    local value=$(echo $x | strip)
+    if [[ ${#value} -eq 0 ]]; then
+      tup Nothing
+    else
+      tup Just "$value"
+    fi
+  fi
+}
+
+maybemap() {
+  local x
+  read x
+  if [[ $(tupl $x) = "Nothing" ]]; then
+    echo $x
+  else
+    local y=$(tupr "$x")
+    local r=$(echo "$y" | map "$@")
+    maybe "$r"
+  fi
+}
+
+maybevalue() {
+  local default="$*"
+  local x
+  read x
+  if [[ $(tupl $x) = "Nothing" ]]; then
+      echo "$default"
+  else
+      echo $(tupr $x)
+  fi
+}
+

test/maybe_test.sh

@@ -0,0 +1,29 @@
+#! /bin/bash
+
+testMaybe() {
+  assertEquals '(Just,1)' "$(maybe 1)"
+  assertEquals '(Just,1)' "$(echo 1 | maybe)"
+  assertEquals '(Nothing)' "$(maybe '')"
+  assertEquals '(Nothing)' "$(maybe '    ')"
+  assertEquals '(Nothing)' "$(maybe '    ' '  ' '   ')"
+  assertEquals '(Nothing)' "$(echo | maybe)"
+  assertEquals '(Just,1 2 3)' "$(maybe 1 2 3)"
+  assertEquals '(Just,1 2 3)' "$(echo 1 2 3 | maybe)"
+}
+
+testMaybemap() {
+  assertEquals '(Just,3)'  "$(echo 1 | maybe | maybemap lambda a . 'echo $(( a + 1 ))' | maybemap lambda a . 'echo $(( a + 1 ))')"
+  assertEquals '(Nothing)' "$(echo   | maybe | maybemap lambda a . 'echo $(( a + 1 ))' | maybemap lambda a . 'echo $(( a + 1 ))')"
+
+  assertEquals '(Nothing)'  "$(echo 1 | maybe | maybemap lambda a . 'echo $(( a + 1 ))' | maybemap lambda a . 'echo')"
+  assertEquals '(Nothing)'  "$(echo 1 | maybe | maybemap lambda a . 'echo $(( a + 1 ))' | maybemap lambda a . 'echo' | maybemap lambda a . 'echo $(( a + 1 ))')"
+}
+
+testMaybevalue() {
+  assertEquals 3 "$(echo 1 | maybe | maybemap lambda a . 'echo $(( a + 1 ))' | maybemap lambda a . 'echo $(( a + 1 ))' | maybevalue 0)"
+  assertEquals 0 "$(echo   | maybe | maybemap lambda a . 'echo $(( a + 1 ))' | maybemap lambda a . 'echo $(( a + 1 ))' | maybevalue 0)"
+  assertEquals 'a b c' "$(echo   | maybe | maybemap lambda a . 'echo $(( a + 1 ))' | maybemap lambda a . 'echo $(( a + 1 ))' | maybevalue a b c)"
+}
+
+
+. ./shunit2-init.sh