haskell - Convert String to Int checking for overflow -

haskell - Convert String to Int checking for overflow -

when tried convert long integer int, surprised no error thrown:

prelude> read "123456789012345678901234567890" :: int -4362896299872285998

readmaybe text.read module gives same result.

two questions:

which function should phone call perform safe conversion? how can type safe language on earth allow such unsafe things?

update 1:

this effort write version of read checks bounds:

{-# language scopedtypevariables #-} parseintegral :: forall . (integral a, bounded a) => string -> maybe parseintegral s = integertointegral (read s :: integer) integertointegral n | n < fromintegral (minbound :: a) = nil integertointegral n | n > fromintegral (maxbound :: a) = nil integertointegral n = $ frominteger n

is best can do?

background: why unchecked overflow wonderful

haskell 98 leaves overflow behavior explicitly unspecified, implementers , bad else. haskell 2010 discusses in 2 sections—in section inherited haskell 98, it's left explicitly unspecified, whereas in sections on data.int , data.word, specified. inconsistency resolved eventually.

ghc kind plenty specify explicitly:

all arithmetic performed modulo 2^n, n number of bits in type.

this extremely useful specification. in particular, guarantees int, word, int64, word32, etc., form rings, , principal ideal rings, under add-on , multiplication. means arithmetic work right—you can transform expressions , equations in lots of different ways without breaking things. throwing exceptions on overflow break these properties, making much more hard write , reason programs. times need careful when utilize comparing operators < , compare—fixed width integers not form ordered groups, these operators bit touchy.

why makes sense not check reads

reading integer involves many multiplications , additions. needs fast. checking create sure read "valid" not easy quickly. in particular, while it's easy find out whether add-on has overflowed, not easy find out whether multiplication has. sensible ways can think of perform checked read int are

read integer, check, convert. integer arithmetic more expensive int arithmetic. smaller things, int16, read can done int, checking int16 overflow, narrowed. cheaper, still not free.

compare number in decimal maxbound (or, negative number, minbound) while reading. seems more reasonably efficient, there still some cost. first section of reply explains, there nil inherently wrong overflow, it's not clear throwing error improve giving reply modulo 2^n.

haskell type-conversion