CODEWITHSUNDEEP
haskelllookup-tablestemplate-haskellcompile-time-constantclash
Cactus
Cactus

Reputation: 27636

Turn a function into a precomputed lookup table without going through Template Haskell

I have the following CLaSH function:

toBCD :: Word8 -> Vec 3 Word8
toBCD x =
    x `div` 100 :>
    (x `div` 10) `mod` 10 :>
    x `mod` 10 :>
    Nil

Of course, the resulting HDL module is unsynthesizable because of the division by a non-power of 2 constant. So I would like to turn it into a lookup table instead, and put it into a small asynchronous ROM.

At first, I thought it would be enough to do something like 

romBCD :: Word8 -> Vec 3 Word8
romBCD = asyncROM $ fmap toBCD $ iterate d256 succ 0

but the resulting VHDL still contains the formula to calculate toBCD 0, toBCD 1, etc. What I would like instead, is to get VHDL which simply contains the precomputed values in a 256x24 table.

One workaround I found was to calculate romBCD's contents via Template Haskell:

romBCD = asyncRom $(listToVecTH $ fmap toBCD [minBound .. maxBound])

but that then forces the definition of romBCD to be in a separate module than toBCD.

Upvotes: 3

Views: 346

Answers (0)

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