natefaubion / purescript-typelevel-eval

Higher order functional programming in PureScript's type system

purescript-typelevel-eval

Higher-order functional programming in PureScript's type system.

Inspired by Haskell with only one type family.

What does it do?

This library exports a typeclass Eval which lets us evaluate type-level expressions of kind TypeExpr. A TypeExpr evaluates to something of kind Type. With this we can do lazy, higher-order functional programming to compute types and their relationships.

Example

Take an input row, and compute a row where all the types are String:

type RowToString =
  ToRow <<< Map (Const String) <<< FromRow

test :: forall ri ro.
  Eval (RowToString ri) ro =>
  { | ri } ->
  { | ro } ->
  Unit
test _ _ = Unit

Assert that all rows have a type String:

type RowAllString =
  Assert "Only String is allowed" <<< All (Eq String) <<< FromRow

test :: forall ri.
  Eval (RowAllString ri) Unit =>
  { | ri } ->
  Unit
test _ = Unit

Emulating Type Families

With Type.Eval.ValueOf we can emulate Haskell's Type Families via explicit coercions.

foreign import data Elem :: Type -> TypeExpr

instance evalElemString :: Eval (Elem String) Char
instance evalElemArray :: Eval (Elem (Array a)) a

testValue :: ValueOf (Elem String)
testValue = ValueOf.from 'a'

test :: Char
test = ValueOf.to testValue

Note that you can use any TypeExpr to tag a ValueOf.