That all depends... In theory all the HList stuff happens at compile
time, and what you are left with is normal function application... Of
course compilers arn't that good yet, but as a reasonable idea, consider
just that value level... Most of the extra work is the packing/unpacking
of pairs (,). I have used HList for database schemas like the Cow
example database (see attached) with no problems. The DB code includes
code to generate the database from this Schema so is doesn't need to
be entered twice, and it also typechecks the database against the schema
in a one-way extensional manner on program start. The performance of the
DB app is good, better than with scripting languages like perl/python,
and type-safe.
This code uses records made from HLists (see the paper for examples).
Keean.
Joel Reymont wrote:
Keean,
I sort of gave up on HList for the time being since I found easier
ways to solve my problem.
Mainly, I could not estimate the impact it would have on run-time
performance of my code and GHC not being able to compile the code was
not a good indication. Simon PJ fixed that error since.
My idea was to, basically, create my own record sans labels. I wanted
to specify picklers and default values for each field instead. I have
over 250 records, though, and some have over 10 fields. There is a
lot of sharing of fields between the records but I still think this
is too much for GHC to handle.
Can you venture a guess on runtime performance of such code?
Thanks, Joel
On Nov 22, 2005, at 4:07 PM, Keean Schupke wrote:
hMarkAll Just hlist
class HList l = HMarkAll c l m | c l - m where
hMarkAll :: (forall a . a - c a) - l - m
instance HMarkAll c HNil HNil where
hMarkAll _ _ = HNil
instance HMarkAll c l m = HMarkAll c (HCons e l) (HCons (c e) m)
where
hMarkAll c (HCons e l) = HCons (c e) (hMarkAll c l)
--
http://wagerlabs.com/
{-# OPTIONS -fglasgow-exts #-}
{-# OPTIONS -fallow-overlapping-instances #-}
{-# OPTIONS -fallow-undecidable-instances #-}
module Lib.Relational.FamDb where
import Char
import Lib.ODBC.Types
import Lib.TIR.HList
import Lib.TIR.HTypeGHC
import Lib.TIR.HRecord
import Lib.Relational.Types as SQL
---
-- Foot and Mouth Database
famdb :: (FarmerTable:*:FarmTable:*:AnimalTable:*:ContaminatedTable:*: HNil)
famdb = (farmerTable.*.farmTable.*.animalTable.*.contaminatedTable.*.HNil)
---
-- Domains
newtype DFarmerId = DFarmerId Int deriving (Show,Eq,ToSqlType SqlInteger,FromSqlType SqlInteger)
newtype DFarmerName = DFarmerName String deriving (Show,Eq,ToSqlType SqlVarchar,FromSqlType SqlVarchar)
newtype DFarmId = DFarmId Int deriving (Show,Eq,ToSqlType SqlInteger,FromSqlType SqlInteger)
newtype DFarmName = DFarmName String deriving (Show,Eq,ToSqlType SqlVarchar,FromSqlType SqlVarchar)
newtype DFarmCounty = DFarmCounty String deriving (Show,Eq,ToSqlType SqlVarchar,FromSqlType SqlVarchar)
newtype DAnimalId = DAnimalId Int deriving (Show,Eq,ToSqlType SqlInteger,FromSqlType SqlInteger)
newtype DAnimalName = DAnimalName String deriving (Show,Eq,ToSqlType SqlVarchar,FromSqlType SqlVarchar)
data DAnimalType = Cow | Sheep deriving (Show,Eq)
newtype DAnimalPrice = DAnimalPrice Float deriving (Show,Eq,ToSqlType SqlNumeric,FromSqlType SqlNumeric)
data DCntdType = BSE | FM deriving (Show,Eq)
instance FromSqlType SqlVarchar DAnimalType where
fromSqlType _ s = case (map toLower s) of
cow - Just Cow
sheep - Just Sheep
_ - Nothing
instance ToSqlType SqlVarchar DAnimalType where
toSqlType Cow = SqlTyped (SqlExpressionConst $ sqlShow cow )
toSqlType Sheep = SqlTyped (SqlExpressionConst $ sqlShow sheep )
instance FromSqlType SqlVarchar DCntdType where
fromSqlType _ s = case (map toLower s) of
bse - Just BSE
fm - Just FM
_ - Nothing
instance ToSqlType SqlVarchar DCntdType where
toSqlType BSE = SqlTyped (SqlExpressionConst $ sqlShow BSE )
toSqlType FM = SqlTyped (SqlExpressionConst $ sqlShow FM )
---
-- Farmer table
data FarmerId = FarmerId deriving Show
data FarmerName = FarmerName deriving Show
type FarmerTable = Table (
FarmerId :=: Attribute DFarmerId SqlInteger :*:
FarmerName :=: Attribute DFarmerName SqlVarchar :*:
HNil)
farmerTable :: FarmerTable
farmerTable = newTable Farmer (
FarmerId .=. Attribute (attr { attrName=farmerid, attrType=SERIAL }) .*.
FarmerName .=. Attribute (attr { attrName=name, attrSize=20 }) .*.
HNil)
---
-- Farm table
data FarmId = FarmId deriving Show
data FarmName = FarmName deriving Show
data FarmCounty = FarmCounty deriving
