:: a -> () -package:ghc-lib

Reduce to weak head normal form Equivalent to \x -> seq x (). Useful for defining NFData for types for which NF=WHNF holds.

data T = C1 | C2 | C3
instance NFData T where rnf = rwhnf

Coerce a value from one type to another, bypassing the type-checker. There are several legitimate ways to use unsafeCoerce:

1. To coerce e.g. Int to HValue, put it in a list of HValue, and then later coerce it back to Int before using it.
2. To produce e.g. (a+b) :~: (b+a) from unsafeCoerce Refl. Here the two sides really are the same type -- so nothing unsafe is happening -- but GHC is not clever enough to see it.
3. In Data.Typeable we have

eqTypeRep :: forall k1 k2 (a :: k1) (b :: k2).
TypeRep a -> TypeRep b -> Maybe (a :~~: b)
eqTypeRep a b
| sameTypeRep a b = Just (unsafeCoerce HRefl)
| otherwise       = Nothing

Here again, the unsafeCoerce HRefl is safe, because the two types really are the same -- but the proof of that relies on the complex, trusted implementation of Typeable.

1. The "reflection trick", which takes advantage of the fact that in class C a where { op :: ty }, we can safely coerce between C a and ty (which have different kinds!) because it's really just a newtype. Note: there is no guarantee, at all that this behavior will be supported into perpetuity.

For safe zero-cost coercions you can instead use the coerce function from Data.Coerce.

Highly, terribly dangerous coercion from one representation type to another. Misuse of this function can invite the garbage collector to trounce upon your data and then laugh in your face. You don't want this function. Really.

The function unsafeCoerce# allows you to side-step the typechecker entirely. That is, it allows you to coerce any type into any other type. If you use this function, you had better get it right, otherwise segmentation faults await. It is generally used when you want to write a program that you know is well-typed, but where Haskell's type system is not expressive enough to prove that it is well typed. The following uses of unsafeCoerce# are supposed to work (i.e. not lead to spurious compile-time or run-time crashes):

• Casting any lifted type to Any
• Casting Any back to the real type
• Casting an unboxed type to another unboxed type of the same size. (Casting between floating-point and integral types does not work. See the GHC.Float module for functions to do work.)
• Casting between two types that have the same runtime representation. One case is when the two types differ only in "phantom" type parameters, for example Ptr Int to Ptr Float, or [Int] to [Float] when the list is known to be empty. Also, a newtype of a type T has the same representation at runtime as T.

Other uses of unsafeCoerce# are undefined. In particular, you should not use unsafeCoerce# to cast a T to an algebraic data type D, unless T is also an algebraic data type. For example, do not cast Int->Int to Bool, even if you later cast that Bool back to Int->Int before applying it. The reasons have to do with GHC's internal representation details (for the cognoscenti, data values can be entered but function closures cannot). If you want a safe type to cast things to, use Any, which is not an algebraic data type. Warning: this can fail with an unchecked exception.

Highly, terribly dangerous coercion from one representation type to another. Misuse of this function can invite the garbage collector to trounce upon your data and then laugh in your face. You don't want this function. Really.

Coerce a value from one type to another, bypassing the type-checker. There are several legitimate ways to use unsafeCoerce:

1. To coerce e.g. Int to HValue, put it in a list of HValue, and then later coerce it back to Int before using it.
2. To produce e.g. (a+b) :~: (b+a) from unsafeCoerce Refl. Here the two sides really are the same type -- so nothing unsafe is happening -- but GHC is not clever enough to see it.
3. In Data.Typeable we have

eqTypeRep :: forall k1 k2 (a :: k1) (b :: k2).
TypeRep a -> TypeRep b -> Maybe (a :~~: b)
eqTypeRep a b
| sameTypeRep a b = Just (unsafeCoerce HRefl)
| otherwise       = Nothing

Here again, the unsafeCoerce HRefl is safe, because the two types really are the same -- but the proof of that relies on the complex, trusted implementation of Typeable.

1. The "reflection trick", which takes advantage of the fact that in class C a where { op :: ty }, we can safely coerce between C a and ty (which have different kinds!) because it's really just a newtype. Note: there is no guarantee, at all that this behavior will be supported into perpetuity.

Warning: this can fail with an unchecked exception.

toFields provides a convenient typeclass wrapper around the Field_ creation functions in Opaleye.SqlTypes. Besides convenience it doesn't provide any additional functionality. It can be used with functions like runInsert to insert custom Haskell types into the database. The following is an example of a function for inserting custom types.

customInsert
:: ( Default ToFields haskells fields )
=> Connection
-> Table fields fields'
-> [haskells]
-> IO Int64
customInsert conn table haskells = runInsert_ conn Insert {
iTable      = table
, iRows       = map toFields haskells
, iReturning  = rCount
, iOnConflict = Nothing
}

In order to use this function with your custom types, you need to define an instance of Default ToFields for your custom types.

Version of toFields with better type inference

Lift r to the answer type, similar to return. This return function aims to be used as the last statement of a do block. When return is present in a nested do block for when or unless, if the r' is not (), it will create a Cont that performs early return, skipping the rest statements of the outer do notation.

Examples

>>> :set -XTypeOperators

>>> :set -XRebindableSyntax

>>> import Prelude hiding ((>>), (>>=), return, fail)

>>> import Control.Dsl

>>> import Control.Dsl.Return

>>> import Control.Dsl.Yield

>>> import Control.Dsl.Cont

>>> import Control.Dsl.Empty

>>> :{
earlyGenerator :: Bool -> Cont [String] Integer
earlyGenerator earlyReturn = do
Yield "inside earlyGenerator"
when earlyReturn $ do
Yield "early return"
return 1
Yield "normal return"
return 0
:}

>>> :{
earlyGeneratorTest :: [String]
earlyGeneratorTest = do
Yield "before earlyGenerator"
i <- earlyGenerator True
Yield "after earlyGenerator"
Yield $ "the return value of earlyGenerator is " ++ show i
empty
:}

>>> earlyGeneratorTest
["before earlyGenerator","inside earlyGenerator","early return","after earlyGenerator","the return value of earlyGenerator is 1"]

Compute the number of segments of anything measured by SegMeasure (e.g. SegMeasure itself, Segment, SegTree, Trails...)

Euclidean norm of a vector.

A utility, which uses the Show instance to produce a value that is isomorphic to String. It lets you generalize over the functions like the following:

showAsText :: Show a => a -> Text
showAsText = showAs @Text

showAsBuilder :: Show a => a -> Builder
showAsBuilder = showAs @Builder