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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">So perhaps you can update the wiki page to give an example like this, and thereby explain the design choice? Or have a FAQ: “why not give TypeErorr the kind String
-> Constraint?”.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">The thought will be lost in email!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Iavor Diatchki [mailto:iavor.diatchki@gmail.com]
<br>
<b>Sent:</b> 17 November 2015 00:10<br>
<b>To:</b> Ben Gamari<br>
<b>Cc:</b> Simon Peyton Jones; Richard Eisenberg; Dominique Devriese; ghc-devs@haskell.org<br>
<b>Subject:</b> Re: Further custom type error questions<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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Hello,<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
I imagine people wanting to do things as in the example below. If we were to use only `TypeError` constraints, then we'd have to mostly use the class system to do type-level evaluation. It doesn't seem obvious how to do that with just `TypeError` of kind
constraint, unless all evaluation was to happen using the class system. <o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
-Iavor<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
PS: Interestingly, this example reveals a bug with GHC's new warning about unused constraints, where the `OffsetOf` constant on `get` is reported as unnecessary... I'll file a bug.<o:p></o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">{-# LANGUAGE TypeFamilies, TypeOperators, UndecidableInstances, DataKinds #-}</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">{-# LANGUAGE ScopedTypeVariables #-}</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">import GHC.TypeLits</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">import Data.Proxy</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">import Data.Word</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">import Foreign.Ptr</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Courier New"">type OffsetOf l xs = GetOffset 0 l xs</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">type family ByteSize x where</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> ByteSize Word64 = 8</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> ByteSize Word32 = 4</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> ByteSize Word16 = 2</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> ByteSize Word8 = 1</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> ByteSize a = TypeError (Text "The type " :<>: ShowType a :<>:</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> Text " is not exportable.")</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Courier New"">type family GetOffset n (l :: Symbol) xs where</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> GetOffset n l ( '(l,a) ': xs) = '(n,a)</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> GetOffset n l ( '(x,a) : xs) = GetOffset (n+ByteSize a) l xs</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> GetOffset n l '[] = TypeError (Text "Missing field: " :<>:</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> ShowType l)</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">newtype Struct (a :: [(Symbol,*)]) = Struct (Ptr ())</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">get :: forall l fs n a.</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> (OffsetOf l fs ~ '(n,a), KnownNat n) =></span><o:p></o:p></p>
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<span style="font-family:"Courier New""> Struct fs -></span><o:p></o:p></p>
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<span style="font-family:"Courier New""> Proxy l -></span><o:p></o:p></p>
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<span style="font-family:"Courier New""> Ptr a</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">get (Struct p) _ = plusPtr p (fromInteger (natVal (Proxy :: Proxy n)))</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">type MyStruct = [ '("A",Word8), '("B",Word8), '("C",Int) ]</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">testOk :: Struct MyStruct -> Ptr Word8</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">testOk s = get s (Proxy :: Proxy "B")</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">{-</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">testNotOk :: Struct MyStruct -> Ptr Word8</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">testNotOk s = get s (Proxy :: Proxy "X")</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">--}</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">{-</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">type MyOtherStruct = [ '("A",Int), '("B",Word8) ]</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">testNotOk :: Struct MyOtherStruct -> Ptr Word8</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">testNotOk s = get s (Proxy :: Proxy "B")</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">--}</span><o:p></o:p></p>
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On Mon, Nov 16, 2015 at 1:21 PM, Ben Gamari <<a href="mailto:ben@smart-cactus.org" target="_blank">ben@smart-cactus.org</a>> wrote:<o:p></o:p></p>
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<br>
While preparing some additional documentation for Iavor's custom type<br>
errors work (which has been merged; thanks Iavor!) I noticed that<br>
Dominique Devriese has raised some additional questions on the proposal<br>
[1].<br>
<br>
In particular, Dominique suggests that perhaps TypeError should simply<br>
be of kind `ErrorMessage -> Constraint`. My understanding of the<br>
proposal is that the intention is that `TypeError`s should be usable on<br>
the RHS of type functions, like `error` on the term level. However, is<br>
this strictly necessary? Is there any place where you couldn't just as<br>
easily express the `TypeError` as a constraint?<br>
<br>
If not, it seems like this may be substantially simpler approach and<br>
totally side-steps the detection of `TypeError` in inappropriate places<br>
on the RHS.<br>
<br>
Regardless, it seems like this (and the other questions) is worth<br>
addressing in the proposal.<br>
<br>
Cheers,<br>
<br>
- Ben<br>
<br>
<br>
[1] <a href="https://ghc.haskell.org/trac/ghc/wiki/Proposal/CustomTypeErrors#SomedesignquestionsDominiqueDevriese" target="_blank">
https://ghc.haskell.org/trac/ghc/wiki/Proposal/CustomTypeErrors#SomedesignquestionsDominiqueDevriese</a>:<o:p></o:p></p>
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