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scala> import scalaz.syntax.bitraverse._
import scalaz.syntax.bitraverse._

scala> import scalaz.std.tuple._
import scalaz.std.tuple._

scala> (List(1,2,3), List("a")).leftMap(_.map(_ + 1))
res0: (List[Int], List[String]) = (List(2, 3, 4),List(a))

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* tags
** load-tags-for-this-project
#+begin_src emacs-lisp
(defun find-file-upwards (file-to-find)
  "Recursively searches each parent directory starting from the default-directory.
looking for a file with name file-to-find.  Returns the path to it
or nil if not found."
  (labels
      ((find-file-r (path)
                    (let* ((parent (file-name-directory path))

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(defun build-package-name (pn d)
  (if (null d)
      pn
    (let ((c (car d)))
      (if (equal c "scala")
	  pn
	(build-package-name (concat c "." pn) (cdr d))))))
 
(defun scala-package-name-from-buffer ()
  (let ((l (reverse (split-string (buffer-file-name) "/"))))

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This tries to find where the Spec/test of the current buffer lives,
and either creates it, or visits it
#+begin_src emacs-lisp
 (defun split-path-of-file (f)
"return dirname.filename" (let ((sp (reverse (split-string f "/"))))
(cons (mapconcat 'identity (reverse (cdr sp)) "/") (car sp))))

(defun scala-test-file-name (f)
  (let* ((sp (reverse (split-string f "\\.")))
	 (h (car sp))

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scala> List(BigDecimal(1).some, BigDecimal(13).some, BigDecimal(12).some).sequence.map(_.sum)
res14: Option[scala.math.BigDecimal] = Some(26)

scala> List(BigDecimal(1).some, None, BigDecimal(12).some).sequence.map(_.sum)
res15: Option[scala.math.BigDecimal] = None

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[debug] Waiting for thread run-main to exit
[error] (run-main) java.lang.AbstractMethodError: spire.example.RandomForestExample$$anonfun$1.apply(Ljava/lang/Object;)Ljava/lang/Object;
java.lang.AbstractMethodError: spire.example.RandomForestExample$$anonfun$1.apply(Ljava/lang/Object;)Ljava/lang/Object;
        at spire.example.CrossValidation$.loop$1(DataSets.scala:234)
        at spire.example.CrossValidation$.crossValidate(DataSets.scala:242)
        at spire.example.CrossValidation$.crossValidateClassification(DataSets.scala:259)
        at spire.example.RandomForestExample$.testClassification(randomforest.scala:40)
        at spire.example.RandomForestExample$delayedInit$body.apply(randomforest.scala:24)
        at scala.Function0$class.apply$mcV$sp(Function0.scala:40)
        at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)

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; scala-mode2 found here: https://github.com/hvesalai/scala-mode2 is much better than the scala mode that comes with scalac
(add-to-list 'load-path "~/.emacs.d/scala-mode2/")
(require 'scala-mode)

; http://www.emacswiki.org/emacs/RainbowDelimiters
(add-hook 'scala-mode-hook 'rainbow-delimiters-mode)

; I get ensime from git:
; https://github.com/aemoncannon/ensime
; then run `sbt stage` inside the ensime source directory,

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scala> val f : Int => List[Int] = List(_)
f: Int => List[Int] = <function1>

scala> Stream(1,2,3) traverse f
res1: List[scala.collection.immutable.Stream[Int]] = List(Stream(1, ?))

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(defun right-arrow ()
  (interactive)
  (cond ((looking-back "=") 
   (backward-delete-char 1) (insert "⇒"))
	((looking-back "-")
	 (backward-delete-char 1) (insert "→"))
	(t (insert ">"))))

(defun left-arrow ()
  (interactive)

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trait UnapplyMT[TC[_[_[_], _]], N[_], MTNA] {
  /** the type constructor */
  type MT[_[_], _]

  /** The type that MT is applied to */
  type A

  /** The instance of the type class */
  def TC: TC[MT]