Troubleshooting

A list of often encountered errors and their solutions:

• Type error: Couldn’t match expected type Signal dom (a,b) with actual type (Signal dom a, Signal dom b):

  Signals of product types and product types of signals are isomorphic due to synchronisity principle, but are not (structurally) equal. Tuples are a product type. Use the bundle function to convert from a product type to the signal type. So if your code which gives the error looks like:

  ... = f a b (c,d)
  

  add the bundle function like so:

  ... = f a b (bundle (c,d))
  

  Product types supported by bundle are:

  • All tuples up to and including 62-tuples (GHC limit)
  • The Vector type

• Type error: Couldn’t match expected type (Signal dom a, Signal dom b) with actual type Signal dom (a,b):

  Product types of signals and signals of product types are isomorphic due to synchronicity principle, but are not (structurally) equal. Tuples are a product type. Use the unbundle function to convert from a signal type to the product type. So if your code which gives the error looks like:

  (c,d) = f a b
  

  add the unbundle function like so:

  (c,d) = unbundle (f a b)
  

  Product types supported by unbundle are:

  • All tuples up to and including 62-tuples (GHC limit)
  • The Vector type

• Clash.Netlist(..): Not in normal form: <REASON>: <EXPR>:

  A function could not be transformed into the expected normal form. This usually means one of the following:

  • The topEntity has higher-order arguments, or a higher-order result.
  • You are using types which cannot be represented in hardware.

  The solution for all the above listed reasons is quite simple: remove them. That is, make sure that the topEntity is completely monomorphic and first-order. Also remove any variables and constants/literals that have a non-representable type; see Limitations of Clash to find out which types are not representable.

• Clash.Normalize(..): Clash can only normalize monomorphic functions, but this is polymorphic:

  If this happens for a topEntity or something with a Synthesize annotation, add a monomorphic type signature. Non topEntites should be type-specialized by clash automatically, if not please report this as a bug. But adding a monomorphic type signature should still help (when possible).

• Clash.Normalize(..): Expr belonging to bndr: <FUNCTION> remains recursive after normalization:

  • If you actually wrote a recursive function, rewrite it to a non-recursive one using e.g. one of the higher-order functions in Clash.Sized.Vector

  • You defined a recursively defined value, but left it polymorphic:

  topEntity x y = acc
    where
      acc = register 3 (acc + x * y)
  

  The above function, works for any number-like type. This means that acc is a recursively defined polymorphic value. Adding a monomorphic type annotation makes the error go away:

  topEntity
    :: SystemClockResetEnable
    => Signal System (Signed 8)
    -> Signal System (Signed 8)
    -> Signal System (Signed 8)
  topEntity x y = acc
    where
      acc = register 3 (acc + x * y)
  

View full listing

• Clash.Normalize.Transformations(..): InlineNonRep: <FUNCTION> already inlined 100 times in:<FUNCTION>, <TYPE>:

  You left the topEntity function polymorphic or higher-order: use :i topEntity to check if the type is indeed polymorphic or higher-order. If it is, add a monomorphic type signature, and/or supply higher-order arguments.

• <*** Exception: <<loop>> or “blinking cursor”

  You are using value-recursion, but one of the Vector functions that you are using is too strict in one of the recursive arguments. For example:

  -- Bubble sort for 1 iteration
  sortV xs = map fst sorted :< (snd (last sorted))
   where
     lefts  = head xs :> map snd (init sorted)
     rights = tail xs
     sorted = zipWith compareSwapL lefts rights
  
  -- Compare and swap
  compareSwapL a b = if a < b then (a,b) else (b,a)
  

View full listing

Will not terminate because `zipWith` is too strict in its second argument.

In this case, adding `lazyV` on `zipWith`s second argument:

```haskell,clash group=troubleshooting
sortVL xs = map fst sorted :< (snd (last sorted))
 where
   lefts  = head xs :> map snd (init sorted)
   rights = tail xs
   sorted = zipWith compareSwapL (lazyV lefts) rights
```

View full listing

Results in a successful computation:

```haskell,clash group=troubleshooting
>>> sortVL (4 :> 1 :> 2 :> 3 :> Nil)
1 :> 2 :> 3 :> 4 :> Nil
```

View full listing

Full code listings

Group troubleshooting

module TroubleshootingExamples where

import Clash.Prelude

topEntity
  :: SystemClockResetEnable
  => Signal System (Signed 8)
  -> Signal System (Signed 8)
  -> Signal System (Signed 8)
topEntity x y = acc
  where
    acc = register 3 (acc + x * y)

-- Bubble sort for 1 iteration
sortV xs = map fst sorted :< (snd (last sorted))
 where
   lefts  = head xs :> map snd (init sorted)
   rights = tail xs
   sorted = zipWith compareSwapL lefts rights

-- Compare and swap
compareSwapL a b = if a < b then (a,b) else (b,a)

sortVL xs = map fst sorted :< (snd (last sorted))
 where
   lefts  = head xs :> map snd (init sorted)
   rights = tail xs
   sorted = zipWith compareSwapL (lazyV lefts) rights