In Haskell we have typeclasses like Real, Integral and IsString for writing polymorphic code with numbers and strings. Would it make sense to have a class Boolish a with a toBool :: a -> Bool function and then modify the if statement so that it could accept a Boolish a instead of a simple Bool? Th…

You might be looking for the RebindableSyntax extension, per which: Conditionals (e.g. “if e1 then e2 else e3”) means “ifThenElse e1 e2 e3”.

I would definitely not like that! Python has something similar through its user-definable per-class __bool__ method and I would not say it leads to clearer code. What’s wrong with just applying a wrapper function?

I’d like to add to this that what defining truthy and falsey values, we must differentiate between notFalse x = x /= false and isTrue x = x == true, for some boolish values of false :: x and true :: x. When languages implicitly cast values to a boolean, they usually use notFalse, and while these two…

Coq allows to use if _ then _ else _ expressions for every data type with exactly two constructors: Extended pattern matching — Coq 8.17.1 documentation But this is more ad-hoc than using a typeclasses and looks directly at the definition of the datatype you are using as the first argument. The mai…

typeclasses like Real, Integral and IsString Notice how all of these typeclasses are unambiguous, every type that deserves an instance only has one possible correct one. Equating something to a boolean on the other hand is completely context dependent. You don’t actually gain anything from a type…

I recently did something akin to that idea using “Rebindable Syntax”. Here is a minimal example: {-# LANGUAGE GADTs #-} {-# LANGUAGE RebindableSyntax #-} import Prelude class IfThenElse a b where ifThenElse :: a -> b -> b -> b -- For some weird eDSL data Expr a where …

Having a polymorphic version of if is useful for embedded DSLs. sbv , for example, has an SBool type with its own versions of if-then-else, == and so on, since it needs to turn if-the-else into logic formulas at runtime rather than taking one branch or the other. I’ve wanted the same kind of functio…

Possibly related? [image] Botan bindings devlog It’s time for another update! Community proposal submission The first thing is something to celebrate - I have officially submitted the cryptography community project proposal to the Haskell Foundation - this p…

Thanks all for your answers! I know that this is not that important, but just for the sake of discussion, let me provide an example: consider a datatype data ShouldShow = Show | Hide if we want to consume a value shouldShow of that type, the most Haskelly way to do it is to use pattern match…

It seems to me that the primary value of OverloadedStrings comes from convenient syntax for introduction forms, that is, string literals. But the compelling DSL use case that @Tikhon identified seems best served by overloaded elimination forms - writing STrue is not so painful, but writing ite A B …

Polymorphic `if` statement

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atravers September 20, 2023, 4:59am 10

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