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What is the purpose of 'pure' keyword in the clash tutorial example?

In Clash official website, there is the following example :

>>> sampleN @System 4 (register 0 (pure (8 :: Signed 8)))

I know what is a pure function, but why this keyword here ? If I remove it I got an error :

Clash.Prelude> sampleN @System 4 (register 0 (8 :: Signed 8))

<interactive>:2:32: error:
    * Couldn't match expected type `Signal System a'
                  with actual type `Signed 8'
    * In the second argument of `register', namely `(8 :: Signed 8)'
      In the third argument of `sampleN', namely
        `(register 0 (8 :: Signed 8))'
      In the expression: sampleN @System 4 (register 0 (8 :: Signed 8))
    * Relevant bindings include it :: [a] (bound at <interactive>:2:1)

Any clue ?

Solution

  • Signal has an instance of Applicative to which pure belongs. pure :: a -> Signal dom a lifts a value of type Signed 8 to Signal dom (Signed 8). Which unifies with Signal System a in that context to result in Signal System (Signed 8).

    Reference used: http://hackage.haskell.org/package/clash-prelude-1.0.0/docs/Clash-Signal.html