CODEWITHSUNDEEP
ats
Pezhead9000
Pezhead9000

Reputation: 31

Resolving ATS type error

Preface this with: learning ATS from the ATS web site.

When trying to pull a record out of a list, once I try to reference the record variable I get a type error on x4.a below.

So why does this fail - wouldn't x4 know it has a record?

typedef
abc1_rec = '{a=int, b=int, c=char}
typedef
abc2_rec = '{a=int, b=char, c=string} 

val x1 = '{a=1,b=3,c='A'} : abc1_rec        // boxed record
val x2 = '{a=1,b='B',c="CAT"} : abc2_rec    // boxed record
val a1 = x2.a                               // =1
val x3 = (x2 :: x1 :: list_nil())           // concat list x2 + x1 + nil
val x4  = x3.head()                         // record x2

val x5 = x4.a      // error - [a] cannot be found

/tmp/patsopt_tcats_lu0auT: 1440(line=77, offs=12) -- 1442(line=77, offs=14): error(3): [a] cannot be found: the type [S2EVar(779)] is expected to be a tyrec(record). patsopt(TRANS3): there are [1] errors in total. exit(ATS): uncaught exception: _2home_2hwxi_2Research_2ATS_2dPostiats_2src_2pats_error_2esats__FatalErrorExn(1025)

Upvotes: 3

Views: 158

Answers (1)

Hongwei Xi
Hongwei Xi

Reputation: 935

Note x1 and x2 are of different types. They cannot really be put into the same list. In this case, the type for x3 is list(X, 2) for some T that is assumed to be a subtype of both abc1_rec and abc2_rec; x4 is of the type T, and x4.a gives you a type-error because the typechecker cannot infer that T is a record type.

Upvotes: 1

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