[Haskell-cafe] Is it possible to type a function to accept only literal values?

[Tom Ellis]

On Tue, Feb 27, 2024 at 12:32:48PM -0500, Daneel Yaitskov wrote:
> the intel manual defines function prototypes as normal:
> 
>    __m128i _mm_bsrli_si128 (__m128i a, int imm8)
> 
> Using a variable as second argument in _mm_bsrli_si128 produces
> unclear, but  compilation error with exact line number.
[...]
> Haskell is known as one of the best languages in type acrobatics, but
> could it beat C here?

Sure, you can use Template Haskell for this.  Use of `wellFormedBsrli`
ensures that its second argument is always evaluated at compile time.
See the output from GHC below for confirmation of this.

Tom


    {-# LANGUAGE TemplateHaskell #-}
    
    module A where
    
    import Language.Haskell.TH
    import Language.Haskell.TH.Syntax
    
    bsrli :: Int -> Int -> Int
    bsrli = error "Implement bsrli here, using FFI, I guess"
    
    wellFormedBsrli :: Quote m => Int -> Code m (Int -> Int)
    wellFormedBsrli y = [|| \x -> bsrli x $$(liftTyped y) ||]
    
    

    {-# LANGUAGE TemplateHaskell #-}
    {-# OPTIONS_GHC -ddump-splices #-}
    
    module B where
    
    import A
    
    example :: Int
    example = $$(wellFormedBsrli (3 + 2)) 0x1
    

    ghc B.hs -ddump-splices
    [1 of 2] Compiling A                ( A.hs, A.o, A.dyn_o ) [Source
    file changed]
    [2 of 2] Compiling B                ( B.hs, B.o, B.dyn_o ) [A[TH]
    changed]
    B.hs:9:5-29: Splicing expression
        wellFormedBsrli (3 + 2) ======> \ x_a2zf -> (bsrli x_a2zf) 5