Databases
StackOverflow2013
Postsdbo
POType inference of instance functions
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POType inference of instance functions
    primarykey
    data
    text
    <p><strong>The Problem</strong></p> <p>I want to be able to create 2 <code>data types</code>: <code>A</code> and <code>B</code> and to create 2 functions <code>f</code>:</p> <ul> <li><code>f :: A -&gt; Int -&gt; Int</code></li> <li><code>f :: B -&gt; String -&gt; String -&gt; String</code></li> </ul> <p>The only way I can do it (so far as I know) is to use <code>type classes</code> and <code>instances</code>.</p> <p>The problem is, that I do not want to explicit write <code>f</code> signatures - I want type checker to infer it for me. Is it possible?</p> <p><strong>Example code</strong></p> <pre><code>{-# LANGUAGE FlexibleInstances, FunctionalDependencies, UndecidableInstances #-} data A = A{ax::Int} deriving(Show) data B = B{bx::Int} deriving(Show) data C = C{cx::Int} deriving(Show) -- I don't want to explicit say the signature is Int-&gt;Int -- I would love to write: -- instance Func_f A (a-&gt;b) where instance Func_f A (Int-&gt;Int) where f _ i = i*2 -- I don't want to explicit say the signature is String-&gt;String-&gt;String -- I would love to write: -- instance Func_f B (a-&gt;b-&gt;c) where instance Func_f B (String-&gt;String-&gt;String) where f _ s1 s2 = "test"++s1++s2 -- I don't want to explicit say the signature is a-&gt;a -- I would love to write: -- instance Func_f C (a-&gt;b) where instance Func_f C (a-&gt;a) where f _ i = i class Func_f a b | a -&gt; b where f :: a -&gt; b f2 _ s1 s2 = "test"++s1++s2 -- Here the type inferencer automaticly recognizes the signature main :: IO () main = do let a = A 1 b = B 2 c = C 3 a_out = f a 5 b_out = f b "a" "b" c_out = c 6 print a_out print b_out print c_out </code></pre> <p><strong>Explaination</strong></p> <p>I'm writing custom domain language compiler and I'm generating Haskell code as a result. I don't want the final users of my language write explicit types, so I want to use Haskells powerful type system to infer as much as possible.</p> <p>If I write function like <code>f2 _ s1 s2 = "test"++s1++s2</code> I do <strong>not</strong> have to explicit write its signature - because compiler can infer it. Can we somehow ask compiler to infer the signatures of <code>f</code> in the above example?</p> <p>I would love to know every possible "hack" to solve this problem, even if this hack would be "ugly", because I'm generating Haskell code and it does not have to be "pretty".</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USWojciech Danilo
    1. USWojciech Danilo
    plurals
    1. 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
    1. VO
      singulars
      1. POType inference of instance functions
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POType inference of instance functions
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POType inference of instance functions
      1. This table or related slice is empty.
      1. VTUpMod
    1. COThere is no way (that I know of) to have the type of a type class parameter inferred from a given implementation of the class. That could be difficult since your first example could be inferred to have type `f :: (Num a) => A -> a -> a`. Will any of the implementations of `f` make use of the first argument or is it just used to determine which version of `f` to use?
      singulars
      1. POType inference of instance functions
      1. USsabauma
    2. CO@sabauma - I know it - but I really do not want to infer very detailed type like `Int->Int` - I would be happy if Haskell would infer it as `f :: (Num a) => A -> a -> a` or something else - if I can later use it (If its even possible). The first argument would be sometimes used - you can think of it like "this" in imperative languages.
      singulars
      1. POType inference of instance functions
      1. USWojciech Danilo
    3. CO@sabauma - I've edited the example - please see the comments.
      singulars
      1. POType inference of instance functions
      1. USWojciech Danilo
 

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