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Desugar string literals in fcases during IL lowering

Fredrik Wieczerkowski requested to merge fix-fcase-string-literals into master

Fixes #145 (closed)

This seems to produce the right FlatCurry, but I would appreciate someone more familiar with the Curry to IL translation to check whether this desugaring is sound. The example from the issue, i.e.

wubbel :: String -> ()
wubbel s = fcase s of
    "Blah" -> ()

now emits

FcaseStringLiteral.curry:2:12-3:16 Warning:
    Pattern matches are non-exhaustive
    In an fcase alternative:
      Patterns not matched:
        []
        (x : _) with x `notElem` ['B']
        ['B']
        ('B' : x : _) with x `notElem` ['l']
        ...
   | 
 2 | wubbel s = fcase s of
   |            ^^^^^^^^^^...

and compiles to

Prog "FcaseStringLiteral" ["Prelude"] [] [Func ("FcaseStringLiteral","wubbel") 1 Public (FuncType (TCons ("Prelude","[]") [TCons ("Prelude","Char") []]) (TCons ("Prelude","()") [])) (Rule [1] (Case Flex (Var 1) [Branch (Pattern ("Prelude",":") [2,3]) (Case Flex (Var 2) [Branch (LPattern (Charc 'B')) (Case Flex (Var 3) [Branch (Pattern ("Prelude",":") [4,5]) (Case Flex (Var 4) [Branch (LPattern (Charc 'l')) (Case Flex (Var 5) [Branch (Pattern ("Prelude",":") [6,7]) (Case Flex (Var 6) [Branch (LPattern (Charc 'a')) (Case Flex (Var 7) [Branch (Pattern ("Prelude",":") [8,9]) (Case Flex (Var 8) [Branch (LPattern (Charc 'h')) (Case Flex (Var 9) [Branch (Pattern ("Prelude","[]") []) (Comb ConsCall ("Prelude","()") []),Branch (Pattern ("Prelude",":") [10,11]) (Comb FuncCall ("Prelude","failed") [])])]),Branch (Pattern ("Prelude","[]") []) (Comb FuncCall ("Prelude","failed") [])])]),Branch (Pattern ("Prelude","[]") []) (Comb FuncCall ("Prelude","failed") [])])]),Branch (Pattern ("Prelude","[]") []) (Comb FuncCall ("Prelude","failed") [])])]),Branch (Pattern ("Prelude","[]") []) (Comb FuncCall ("Prelude","failed") [])]))] []

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