小编Joe*_*Joe的帖子

data Suit = Spade | Heart | Club | Diamond deriving (Eq, Ord, Enum, Show)

data Pip = Ace | Two | Three | Four | Five | Six | Seven | Eight | Nine    | Ten | Jack | Queen | King deriving (Eq, Ord, Enum, Show)

data Card = Card
       {suit :: Suit
       ,pip :: Pip
       } deriving (Eq,Ord, Show)

type Deck = [Card]

data EOBoard = EOBoard
         {foundations :: [Deck]
         ,columns     :: [Deck]
         ,reserve    :: [Deck]
         } …

data Nat = Zero | Succ Nat
type Predicate = (Nat -> Bool)

-- forAllNat p = (p n) for every finite defined n :: Nat

implies :: Bool -> Bool -> Bool
implies p q = (not p) || q 

basecase :: Predicate -> Bool
basecase p = p Zero 

jump :: Predicate -> Predicate
jump p n = implies (p n) (p (Succ n)) 

indstep :: Predicate -> Bool
indstep p = forallnat (jump p) 

题:

证明如果basecase …

我在终端中命令将数据库中的列更改为非空，但它似乎不起作用。

rails g migration change_column_null :Speaker, :surname, false

我得到了一个文件 ChangeColumnNull 但在里面，它什么都不是。

class ChangeColumnNull < ActiveRecord::Migration
def change
end
end

讲师（Def Create）：

class CplecturesController < ApplicationController
layout 'cp_layout'

 def create
@lecture = Lecture.new(lecture_params)

@lecture.save
redirect_to @lecture
end

private
def lecture_params
  params.require(:lecture).permit(:lecture_title, :lecture_day,   :column, :start_time, :end_time, :registered_speakers, :guest_speakers, :description)
end
end

形式

 <%= form_for :lecture, url:lectures_path do |f| %>
 <form>
 <div class="form-group">
  <%=label_tag "Lecture Title" %><br>
  <%= f.text_field :lecture_title, :class => "form-control",   :placeholder => "Example: Why is Wordpress the best?" %>
</div>