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Add coerceExpr, which coerces between Coercible types #373

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JackKelly-Bellroy wants to merge 1 commit into from
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Add coerceExpr, which coerces between Coercible types #373

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3 changes: 2 additions & 1 deletion src/Rel8.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -169,6 +169,7 @@ module Rel8
, Expr
, Sql
, litExpr
, coerceExpr
, unsafeCastExpr
, unsafeCoerceExpr
, unsafeLiteral
Expand Down Expand Up @@ -417,7 +418,7 @@ import Rel8.Expr.Default
import Rel8.Expr.Eq
import Rel8.Expr.Function
import Rel8.Expr.Null
import Rel8.Expr.Opaleye (unsafeCastExpr, unsafeCoerceExpr, unsafeLiteral, unsafePrimExpr)
import Rel8.Expr.Opaleye (coerceExpr, unsafeCastExpr, unsafeCoerceExpr, unsafeLiteral, unsafePrimExpr)
import Rel8.Expr.Ord
import Rel8.Expr.Order
import Rel8.Expr.Serialize
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19 changes: 19 additions & 0 deletions src/Rel8/Expr/Opaleye.hs
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Expand Up @@ -9,6 +9,7 @@
module Rel8.Expr.Opaleye
( castExpr, unsafeCastExpr
, scastExpr, sunsafeCastExpr
, coerceExpr
, unsafeCoerceExpr
, unsafePrimExpr
, unsafeLiteral
Expand All @@ -19,6 +20,7 @@ where

-- base
import Prelude
import Data.Coerce ( Coercible )

-- opaleye
import qualified Opaleye.Internal.Column as Opaleye
Expand All @@ -39,6 +41,23 @@ castExpr :: Sql DBType a => Expr a -> Expr a
castExpr = scastExpr typeInformation


-- | Change the type of an 'Expr' without using @CAST()@, if the
-- Haskell type can be safely coerced.
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@TeofilC TeofilC May 7, 2025

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I'm not sure if it follows that if the Haskell types can be safely coerced then the DB types can be safely coerced. Could you not have a newtype which uses a different DBType?

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@JackKelly-Bellroy JackKelly-Bellroy May 8, 2025

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I think it's sound only if you ever newtype-derive DBType instances. If that's true then it might not pull its weight: either it'll sound safer than it is, or we'll have to call it something like sometimesButNotAlwaysSafeCoerceExpr. Or use some constraint that indicates that it's specifically a newtype and not just Coercible. (e.g. the one in generic-data.)

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@TeofilC TeofilC May 8, 2025

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sometimesButNotAlwaysSafe is exactly what unsafe means though. unsafe just means that there is some pre-condition that we couldn't express in the types so you should be careful when using it

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@JackKelly-Bellroy JackKelly-Bellroy May 8, 2025

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Yeah that's my point. It's slightly less unsafe but still unsafe, which makes me doubt if it's enough of an improvement over unsafeCoerceExpr to include. I might just close this PR.

--
-- This is useful for writing function that use @nextval()@ over @newtype@'d IDs:
--
-- @
-- newtype AuthorId = AuthorId Int64
-- deriving newtype (DBType, DBEq)
--
-- nextId :: Expr AuthorId
-- nextId = coerceExpr $ nextVal "authors_id_seq"
-- @
--
coerceExpr :: forall b a. Coercible a b => Expr a -> Expr b
coerceExpr = unsafeCoerceExpr


-- | Cast an expression to a different type. Corresponds to a @CAST()@ function
-- call.
unsafeCastExpr :: forall b a. Sql DBType b => Expr a -> Expr b
Expand Down