Databases
StackOverflow2013
Postsdbo
POType constraints for automatic function constraint deduction in Haskell
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POType constraints for automatic function constraint deduction in Haskell
    primarykey
    data
    text
    <p>For educational purposes I am playing around with trees in Haskell. I have <code>Tree a</code> type defined like this</p> <pre><code>data Tree a = EmptyTree | Node a (Tree a) (Tree a) </code></pre> <p>and a lot of functions that share a basic constraint - <code>Ord a</code> - so they have types like</p> <pre><code>treeInsert :: Ord a =&gt; a -&gt; Tree a -&gt; Tree a treeMake :: Ord a =&gt; [a] -&gt; Tree a </code></pre> <p>and so on. I can also define <code>Tree a</code> like this</p> <pre><code>data Ord a =&gt; Tree a = EmptyTree | Node a (Tree a) (Tree a) </code></pre> <p>but I can not simplify my functions and omit the extra <code>Ord a</code> to be as the following:</p> <pre><code>treeInsert :: a -&gt; Tree a -&gt; Tree a treeMake :: [a] -&gt; Tree a </code></pre> <p>Why does Haskell (running with <code>-XDatatypeContexts</code>) not automatically deduce this constraint? It seems to to be quite obvious for me that it should. Why am I wrong? </p> <p>Here is some example source code</p> <pre><code>data (Eq a, Ord a) =&gt; Tree a = EmptyTree | Node a (Tree a) (Tree a) treeInsert :: a -&gt; Tree a -&gt; Tree a treeInsert a EmptyTree = Node a EmptyTree EmptyTree treeInsert a node@(Node v left right) | a == v = node | a &gt; v = Node v left (treeInsert a right) | a &lt; v = Node v (treeInsert a left) right mkTree :: [a] -&gt; Tree a mkTree [] = EmptyTree mkTree (x:xs) = treeInsert x (mkTree xs) </code></pre> <p>I am getting this</p> <pre><code>Tree.hs:5:26: No instance for (Ord a) arising from a use of `Node' In the expression: Node a EmptyTree EmptyTree In an equation for `treeInsert': treeInsert a EmptyTree = Node a EmptyTree EmptyTree Failed, modules loaded: none. </code></pre>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USAndrewC
    1. UStomas789
    plurals
    1. PL
      singulars
      1. LTLinked
    2. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POType constraints for automatic function constraint deduction in Haskell
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POType constraints for automatic function constraint deduction in Haskell
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POType constraints for automatic function constraint deduction in Haskell
      1. This table or related slice is empty.
      1. VTUpMod
    1. COhttp://stackoverflow.com/questions/7438600/datatypecontexts-deprecated-in-latest-ghc-why seems relevant.
      singulars
      1. POType constraints for automatic function constraint deduction in Haskell
      1. USraymonad
    2. COraymonad's link contains what you're looking for. ["GADT-style declarations differ in one important respect: they treat class constraints on the data constructors differently. Specifically, if the constructor is given a type-class context, that context is made available by pattern matching."](http://www.haskell.org/ghc/docs/7.0.3/html/users_guide/data-type-extensions.html#gadt-style)
      singulars
      1. POType constraints for automatic function constraint deduction in Haskell
      1. USphg
    3. COThere's no need for both `Eq` and `Ord` in a context. `Eq` is a superclass of `Ord`, so the latter is enough.
      singulars
      1. POType constraints for automatic function constraint deduction in Haskell
      1. USaugustss
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload