Questions tagged [polymorphic-functions]
28 questions
19
votes
5 answers
Can I print in Haskell the type of a polymorphic function as it would become if I passed to it an entity of a concrete type?
Here's a function polymorphic in 3 types:
:t (.)
(.) :: (b -> c) -> (a -> b) -> a -> c
and here a non polymorphic function:
:t Data.Char.digitToInt
Data.Char.digitToInt :: Char -> Int
If we apply the former to the latter, we get a function…
Enlico
- 23,259
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13
votes
3 answers
Are polymorphic functions "restrictive" in Scala?
In the book Functional Programming in Scala MEAP v10, the author mentions
Polymorphic functions are often so constrained by their type that they only have one implementation!
and gives the example
def partial1[A,B,C](a: A, f: (A,B) => C): B => C =…
Somasundaram Sekar
- 5,244
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8
votes
1 answer
How do I write higher-order functions that take polymorphic functions as arguments in Typed Racket?
For example, how can I write a version of map that will work with polymorphic functions in Typed Racket? I use a simple id function defined as:
(: id : (All (A) A -> A))
(define (id x) x)
When I try to map it over a list i get an error:
> (map id…
Danil Gaponov
- 1,413
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7
votes
1 answer
Dynamically parametrize Poly1 function in shapeless
I have this situation (stripped down to the essential parts)
class Foo[L <: HList](columns: L) {
class toRecord(row: Row) extends Poly1 {
implicit def caseColumn[T] = at[Column[T]] { /* map to a record field */ }
}
def asRecord = {
…
Gabriele Petronella
- 106,943
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4
votes
2 answers
Function parameter anyelement, PostgreSQL bug?
I do not see the bug in this implementation:
CREATE FUNCTION foo(anyelement) RETURNS SETOF int AS $f$
SELECT id FROM unnest(array[1,2,3]) t(id)
WHERE CASE WHEN (pg_typeof($1)::text)='integer' THEN $1::int>2 ELSE true END
$f$ LANGUAGE SQL…
Peter Krauss
- 13,174
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4
votes
3 answers
OCaml Explicit polymorphic type annotations
I would enjoy to receive some helpful comments concerning an example given on:
http://caml.inria.fr/pub/docs/manual-ocaml-400/manual021.html#toc79
7.12 Explicit polymorphic type annotations
type 'a t = Leaf of 'a | Node of ('a * 'a) t
let rec…
user3351512
- 51
- 4
4
votes
2 answers
How do polymorphic inline caches work with mutable types?
A polymorphic inline cache(PIC) works by caching the actual method by the type of the object, in order to avoid the expensive lookup procedures (usually a hashtable lookup).
How does one handle the type comparison if the type objects are mutable…
user85408
- 59
- 1
3
votes
2 answers
In Scala, how to summon a polymorphic function applicable to an input type, without knowing the output type or full type arguments?
Since Scala 2.12 (or is it 2.13, can't be sure), the compiler can infer latent type arguments across multiple methods:
def commutative[
A,
B
]: ((A, B) => (B, A)) = {???} // implementing omitted
val a = (1 -> "a")
val b =…
tribbloid
- 4,026
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3
votes
1 answer
Mapping over generic tuples with polymorphic functions
Scala 3 provides polymorphic functions and Tuples similar to shapeless HList:
scala> 1 *: "foo" *: Tuple()
val res0: (Int, String) = (1,foo)
scala> val f: ([T] => T => Option[T]) = [T] => (v: T) => Some(v)
val f: PolyFunction{apply: [T](x$1: T):…
Mario Galic
- 47,285
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3
votes
1 answer
port Oracle decode() using variadic, anyarray, and anyelement
I need to port from Oracle a stored procedure that uses decode() extensively. That is, I can't use series of CASE WHEN expr THEN expr [...] ELSE as the guide suggests.
I wanted to create a variadic function, but here's the problem: in Oracle the…
basin
- 3,949
- 2
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- 63
3
votes
2 answers
Pattern match on type of polymorphic parameter - alternatives
Let's say I need different output depending on the type of the polymorphic parameter of a function. My initial attempt fails with some cryptic error message:
choice :: a -> Int
choice (_ :: Int) = 0
choice (_ :: String) = 1
choice _ = 2
However, we…
mucaho
- 2,119
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- 35
3
votes
0 answers
Diverging implicit expansion for shapeless LeftFolder
I'm trying to solve this problem and want to fold over an HList using a Poly2 function.
Here's a self-contained MWE (you can copy-paste this inside a REPL with shapeless 2.0 in order to reproduce the issue)
import shapeless._; import…
Gabriele Petronella
- 106,943
- 21
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1
vote
1 answer
Context bounds for polymorphic function type in Scala3?
The set up:
I'm somewhat new to Scala (specifically, Scala 3) - so I understand that the following question might be dumb/naïve.
I know that a polymorphic method type below works.
def zeroMethod[A:Numeric] = Numeric[A].zero
Here Numeric is the…
David Reynolds
- 21
- 4
1
vote
1 answer
Defining a constructor for a typeclass that takes a method with a type parameter?
I have a situation where none of the solutions that I'm aware of feel like good ones. I am trying to define a typeclass, like the example below, where it has an abstract type S that must implement another typeclass (not shown) Valid[A]. It feels…
Chris J Harris
- 1,597
- 2
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- 26
1
vote
1 answer
Type lambda with higher kind
In Dotty given the following:
object Domain {
final case class Create(name: String) extends BaseCreate[Create] {
override type Model = Domain
override def service[F[_]](client: KeystoneClient[F]): CrudService[F, Domain, Create] =…
Simão Martins
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