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Today's Topics:
1. Showing Types (D?niel Arat?)
2. Re: Showing Types (David McBride)
----------------------------------------------------------------------
Message: 1
Date: Mon, 7 Mar 2016 19:06:03 +0100
From: D?niel Arat? <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] Showing Types
Message-ID:
<CAHvKd2+oRn29=X9X7pcFjL23oP0Uq8W59c=9vffn1wl8irs...@mail.gmail.com>
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How does GHCi show types?
Is there a magic function (showType :: a -> String) that does that? Or
is this feature buried somewhere deep in the compiler?
Can I show, compare and reason about types in a Haskell program?
This would be cool:
```haskell
test = do
let len = sum . map (const 1)
when (isInfixOf "Integer" (showType len)) $ putStrLn "restrictive type
inferred; maybe try turning off monomorphism restriction"
```
------------------------------
Message: 2
Date: Mon, 7 Mar 2016 13:27:31 -0500
From: David McBride <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Showing Types
Message-ID:
<CAN+Tr40xCyNuCEjLNb=yczidpamsbzc64ms0xz221qn9a6d...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
You can use the Data.Typeable module
import Data.Typeable
:t typeOf True
typeOf True :: TypeRep
:t show (typeOf True)
show (typeOf True) :: String
typeOf 1 == typeOf 1
True
>typeOf 1 == typeOf True
False
But it works on concrete types, not functions. Sorry.
let len = sum . map (const 1)
typeOf len
<interactive>:17:1:
No instance for (Typeable b0) arising from a use of ?typeOf?
In the expression: typeOf len
In an equation for ?it?: it = typeOf len
On Mon, Mar 7, 2016 at 1:06 PM, D?niel Arat? <[email protected]> wrote:
> How does GHCi show types?
>
> Is there a magic function (showType :: a -> String) that does that? Or
> is this feature buried somewhere deep in the compiler?
>
> Can I show, compare and reason about types in a Haskell program?
>
> This would be cool:
>
> ```haskell
> test = do
> let len = sum . map (const 1)
> when (isInfixOf "Integer" (showType len)) $ putStrLn "restrictive type
> inferred; maybe try turning off monomorphism restriction"
> ```
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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