#2875: Correct SYB's representation of Char
-----------------------------+----------------------------------------------
Reporter: dreixel | Owner:
Type: proposal | Status: new
Priority: normal | Component: libraries/base
Version: 6.10.1 | Severity: normal
Keywords: | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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SYB uses `DataRep` to represent datatypes:
{{{
-- | Public representation of datatypes
data DataRep = AlgRep [Constr]
| IntRep
| FloatRep
| StringRep
| NoRep
}}}
I believe that `StringRep` should be `CharRep`. Note that `IntRep` is used
for the primitive `Int` and `Integer` datatypes, `FloatRep` for `Float`
and `Double`, and `StringRep` (apparently) for `Char`. `String`, however,
is represented as `AlgRep [[],(:)]`:
{{{
*Main> dataTypeOf 'p'
DataType {tycon = "Prelude.Char", datarep = StringRep}
*Main> dataTypeOf "p"
DataType {tycon = "Prelude.[]", datarep = AlgRep [[],(:)]}
}}}
This makes sense, since `String` is not a primitive datatype. But it
causes the apparently wrong behavior:
{{{
*Main> fromConstr (mkStringConstr (dataTypeOf "a") "ab") :: String
"*** Exception: mkStringConstr
*Main> fromConstr (mkStringConstr (dataTypeOf 'a') "ab") :: String
"*** Exception: constrIndex
}}}
The correct way of using `mkStringConstr` is to construct a `Char`. This,
however, only works for strings with a single character:
{{{
*Main> fromConstr (mkStringConstr (dataTypeOf 'a') "b") :: Char
'b'
*Main> fromConstr (mkStringConstr (dataTypeOf 'a') "ab") :: Char
*** Exception: gunfold
*Main> fromConstr (mkStringConstr (dataTypeOf 'a') "") :: Char
*** Exception: gunfold
}}}
I find this behavior counterintuitive. Therefore I propose to rename
`StringRep` to `CharRep` and `mkStringConstr` to `mkCharConstr`. For
backwards compatibility, this entails:
* Deprecating `mkStringConstr` and `StringConstr`
* Deprecating `mkStringRep` and `StringRep`
* Introducing `mkCharConstr` and `CharConstr`
* Introducing `mkCharRep` and `CharRep`
Additionally, due to deprecation warnings, the following have to change as
well:
* libraries/template-haskell/Language/Haskell/TH/Quote.hs
* compiler/utils/Serialized.hs
I'm attaching a patch with the proposed changes. I propose a discussion
period of 4 weeks, therefore until the 8th of January.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2875>
GHC <http://www.haskell.org/ghc/>
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