Can't bind with ScopedTypeVariables in a let

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2020-10-12 09:37:23

    • You cannot bind scoped type variable ‘a’
        in a pattern binding signature

why tho?

Georgi Lyubenov // googleson78

2020-10-12 09:37:39

let (smth :: Smth a) = ...
 in ...

Georgi Lyubenov // googleson78

2020-10-12 09:38:05

help :bow: @Lysxia

TheMatten

2020-10-12 09:51:33

Try case - I think there were some technicalities behind that decision

Georgi Lyubenov // googleson78

2020-10-12 09:51:53

yeah, but I'm interested in why

Georgi Lyubenov // googleson78

2020-10-12 09:52:17

if you have a link handy?

TheMatten

2020-10-12 09:53:49

https://stackoverflow.com/questions/46077442/why-do-haskells-scoped-type-variables-not-allow-binding-of-type-variables-in-pa

Why do Haskell's scoped type variables not allow binding of type variables in pattern bindings?

I noticed that GHC's ScopedTypeVariables is able to bind type variables in function patterns but not let patterns. As a minimal example, consider the type data Foo where Foo :: Typeable a => ...

Georgi Lyubenov // googleson78

2020-10-12 09:55:43

thanks for using google in my stead!

Georgi Lyubenov // googleson78

2020-10-12 09:55:46

:grinning_face_with_smiling_eyes: sorry

Georgi Lyubenov // googleson78

2020-10-12 09:56:05

honestly wasn't expecting to find an answer so I didn't even bother..

TheMatten

2020-10-12 10:04:15

I additionally found this (see 5))

Can't bind with ScopedTypeVariables in a let - Haskell